Modern Optical Spectroscopy X-Ray Microanalysis Shu-Ping Lin, Ph.D. Institute of Biomedical Engineering E-mail: splin@dragon.nchu.edu.tw Website: http://web.nchu.edu.tw/pweb/users/splin/
Backscattered Electron Image (BEI) When Z of atom increases, the image is brighter. ZnO Zn 7 Sb 2 O 12 ZnO
High Vacuum: Need Conductivity! Above left: uncoated. A charge builds up causes oversaturation (white) and horizontal streaking from beam Above right: what same area would appear with conductive coating Right: High vacuum carbon coater ( evaporator not sputterer)
SEM Resolution Tradition: insert an object with a sharp edge that produces high contrast relative to the background, crank up the mag and measure the distance (red) where the SE (secondary electron) signal changes from 10-90% maximum contrast difference. Lyman et al, 1990, Lehigh Lab Workbook, Fig 2.2, p. 11
SEM Resolution The above 2 scans show the technique (though apparently the proofreader didn t catch the inverted signal on the left image). The only way to increase resolution is to turn DOWN the beam current, as the plot on the right shows dramatically -- the current is a few tens of picoamps, not nanoamps. Lyman et al, 1990, Lehigh Lab Workbook, Figs A2.3, A2.4, p.
SEM Resolution However, the approach apparently used today (e.g. our Hitachi field service engineer) is to take his test sample (gold sputtered on graphite substrate) and with optimized contrast, find the narrowest spacing between 2 gold blobs and define that as the resolution.
Depth of Field A strength of the SEM is the enhanced depth of field compared to optical microscopy as shown above for the radiolarian Trochodiscus longispinus. Optical image has only a few micron depth of field (=plane in focus), whereas SEM images can be made to be in focus for hundreds of microns (e.g. increasing working distance) Goldstein et al 2003 Fig 1.3
Stigmatism Imperfect magnetic lenses (metal machining, electrical windings, dirty apertures) can cause the beam to be not exactly round, but astigmatic. This can be corrected using a stigmator, a set of 8 electromagnetic coils (bottom image). Top left: original poor image Top right: underfocus with stig Bottom left: overfocus with stig Bottom right: image corrected for astigmatism. Marker = 200 nm Goldstein et al 2003 Fig 2.24
Edge Effects Edges of objects can appear to be brighter in SE and BSE images, because electrons can be emitted not only from the top but the side, artifically making that part of the image brighter. This can lead to some incorrect conclusions for BSE images. Reed 2005 Fig 4.3
SE imaging: the signal is from the top 5 nm in metals, and the top 50 nm in insulators. Thus, fine scale surface features are imaged. The detector is located to one side, so there is a shadow effect one side is brighter than the opposite. Everhart-Thornley detector: low-energy secondary electrons are attracted by +200 V on grid and accelerated onto scintillator by +10 kv bias; light produced by scintillator (phosphor surface) passes along light pipe to external photomultiplier (PM) which converts light to electric signal. Back scattered electrons also detected but less efficiently because they have higher energy and are not significantly deflected by grid potential. Secondary electron images
Crystal lattice shown above, with 2 beam-crystal orientations: (a) non-channeling, and (b) channelling. Less BS electrons get From Newbury et al, 1986, Advanced Scanning Electron Microscopy and X-ray Microanalysis, Plenum, p. 88 and 159. Variations on a theme There are several alternative type SEM images sometimes found in BSE or SE imaging: (left) channeling (BSE) and (right) magnetic contrast (SE). I have found BSE images of single phase metals with crystalline structure shown by the first effect, and suspect the second effect may be the cause of problems with some Mn-Ni phases.
When electrons hit matter..
When electrons hit matter.. (1) they may collide with an inner shell electron, ejecting same > the ejected electron is a low-energy, secondary electron - detected & used to from SEM images > the original high-energy electron is scattered - known as a back-scattered electron (SEM use) > an outer-shell electron drops into the position formerly occupied by the ejected electron > this is a quantum process, so a X-ray photon of precise wavelength is emitted - basis for X-ray microanalysis
When electrons hit matter..
When electrons hit matter.. (2) they may collide or nearly collide with an atomic nucleus > undergo varying degree ofdeflection (inelastic scattering) > undergo loss of energy - again varying > lost energy appears as X-rays of varying wavelength > this X-ray continuum is identical to that originating from an X-ray source/generator (medical, XRC etc) > original electrons scattered in a forward direction will enter the imaging system, but with wrong l > causes a haze and loss of resolution in image
When electrons hit matter..
When electrons hit matter.. (3) they may collide with outer shell electrons > eithe removing or inserting an electron > results in free radical formation > this species is extremely chemically active > reactions with neighbouring atoms induce massive change in the specimen, especially in the light atoms > this radiation damage severely limits possibilities of EM > examination of cells in the live state NOT POSSIBLE > all examinations need to be as brief (low dose) as possible
When electrons hit matter..
When electrons hit matter.. (4) they may pass through unchanged > these transmitted electrons can be used to form an image > this is called imaging by subtractive contrast > can be recorded by either (a) TV-type camera (CCD) - very expensive (b) photographic film - direct impact of electrons Photographic film > silver halide grains detect virtually every electron > at least 50x more efficient than photon capture!
When electrons hit matter.. beam damage occurs: light elements (H, O) lost very rapidly change in valency shell means free radicals formed...& consequent chemical reactions causing further damage beam damage is minimised by use of low temperatures (-160 ) high beam voltages minimal exposure times
Electron Probe Microanalysis Technique to quantitatively analyze samples for their chemical composition on a micro-scale (~1μm) Instrument Known as probe, microprobe or electron microprobe all the same Main components Electron Gun Beam Column Sample Holder/Air Lock X-ray Spectrometers Wavelength or Energy Dispersive Vacuum System Illumination System Brief Description of Main Components Electron Gun Source of electrons by thermionic emission of Tungsten filament Grid cap (Wehnelt cylinder) negatively biased to constrict electron beam
Beam Column Serves to de-magnify image of filament to achieve ~ 1 m beam size Contains a condenser lens and an objective lens
Sample holder/air lock Provides a means of holding a sample for examination typically a polished thin section or mineral grain mount Air lock provides a means to exchange samples the most dangerous action when working on a probe - mess up and you can back flush diffusion pump oil into the probe column and be banished from the probe forever! X-ray spectrometers Two kinds of spectrometer found on probes wavelength (WDS) and energy dispersive (EDS) WDS 10X sensitivity of EDS Measure the same thing characteristic spectra of an element present in the sample E = hc/λ = 12.396/ λ, E = energy in Kev, h = Planck s constant, c = velocity of light, λ = wavelength in angstroms This relationship shows energy and wavelength are inversely related Method utilizes the characteristic radiation from an excited atom to quantify the amount of an element in a sample Peak height proportional to amount of element present, after background correction Example: a K shell electron is ejected and an L electron drops to fill vacancy leads to a Kα line (this radiation is specific to the atom excited)
Quantitative Analysis To quantify the amount of an element in a sample must compare signal from sample to that of a known standard To first order, counts from sample and counts from standard are directly related to concentration k ratio = I sample /I std = C sample In practice, k ratio must be corrected for sample effects k ratio = I sample /I std = C sample Z A F ZAF means we have to make three kinds of correction to our sample data Z is the so called atomic number correction is made up of stopping power and backscatter terms A is the absorption correction takes into account that some of the X-rays produced in sample volume don t make it out of the sample F is the fluorescence correction corrects for X-ray induced excitation in the sample There is an alternate correction process utilized by many of the modern probes it is the Phi rho-z method, it basically combines the Z and A effects into one method, you still need to do the fluorescence correction as well Software packages included with all modern probes make the corrections for you
Sample - Electron Beam Interaction:
Vacuum System Required to sustain tungsten filament and stop residual gas X-ray absorption Illumination System Two modes usually provided 1. Optical illuminator reflected light, sometimes transmitted light, for visual identification of sample features 2. SEM mode the probe can function as an SEM (scanning electron microscope) with both secondary and backscatter images available extremely useful method for selecting spots to probe Helpful Hints Beam energy must be 1 to 2 times energy required to excite desired X-ray line Beam current should be as low as possible for minimum spot size Spot size can be varied if sample is reactive under the electron beam (be sure to use same spot size for standards)