Electron probe microanalysis EPMA Acquiring Images in the SEM Modified 9/18/09 Modified 10/3/16

“A picture is worth a thousand words”. What’s the point? “A picture is worth a thousand words”. The more we know about how images are acquired, the better we can present research results graphically.

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). This is the old original version with oil diffusion pump (now replaced with turbo).

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. (or 14-86%) 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. 191

SEM Resolution: K409 We found a “failed” NIST-standard that makes a great resolution standard for FE-microprobes. Peter Sobol wrote a program that automatically calculates the beam resolution.

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

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 Stigmatism 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. Note: this explanation is for topography. Reed 2005 Fig 4.3

Secondary electron images 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. (image and text from Reed, 1996, p. 37) 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.

SE1 and SE2 The picture gets a little more complicated … secondary electrons in fact can be generated and detected from more than just the “landing point” of the E0 beam (SE1’s). As the electron scatters away from the impact spot, if it stays near the surface, a second generation (SE2’s) can be generated and detected -- these will cause the SE image to be less sharp. Solution: go to lower E0 (5 kV or less is mentioned by Goldstein) Goldstein et al 2003 Fig 3.20

SE1 and SE2…and SE3! And in the real world it may be even more complicated … backscattered electrons may bounce off the chamber walls, or the bottom of the column, generating SE3’s. Because the E-T detector has a positive bias to attract the low energy SEs, these extraneous signals can add more noise to the image. This is especially problematic at high E0s and is a reason not to expect high resolution SE images at high kV values. Goldstein et al 2003 Fig 4.20

BSE images There are several different types of detectors used to acquire BSE images: Everhart-Thornley detector can have a -50 eV bias put on the grid to reject secondary electrons, so only BSEs get thru -- however this is not useful at fast, TV scanning rates, i.e. moving the stage. Robinson detector — a modern version of the E-T for BSE at TV rates. Must be inserted and retracted. Solid state detector — which is most commonly used today on electron microprobes and many SEMs. Permanently mounted below pole piece. BSE imaging: the signal comes from the top ~.1 um surface). Above, 5 phases stand out in a volcanic ash fragment

BSE images A solid-state (semi-conductor) backscattered electron detector (a) is energized by incident high energy electrons (~90% E0), wherein electron-hole pairs are generated and swept to opposite poles by an applied bias voltage. This charge is collected and input into an amplifier (gain of ~1000). (b) It is positioned directly above the specimen, surrounding the opening through the polepiece. In our SX51 BSE detector, we can modify the amplifier gain: BSE GMIN or BSE GMAX. BSE imaging: the signal comes from the top ~.1 um surface; solid-state detector is sensitive to light (and red LEDs). Goldstein et al, 1992, Fig 4.24, p. 184

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. Crystal lattice shown above, with 2 beam-crystal orientations: (a) non-channeling, and (b) channelling. Less BS electrons get out in B, so darker. From Newbury et al, 1986, Advanced Scanning Electron Microscopy and X-ray Microanalysis, Plenum, p. 88 and 159.

BSE and SE Detectors on our SX51 Annular BSE detectors Anti-contamination air jet Plates for +voltage for SE detector View from inside, looking up obliquely (image taken by handheld digital camera)

Hitachi SEM Detectors Annular BSE detector EDS detector ESED detector E-T SE detector IR Chamberscope View from inside, looking up obliquely (image taken by handheld digital camera)

Mosaic Images There are occasions where the feature you wish to image is larger than the field of view acquirable by the rastered beam. A complete thin section (24x48 mm) can have a mosaic BSE image acquired in < 1 hour (though an X-ray map could take a week, so only smaller areas are typically X-ray mapped.) This is achieved by tiling or mosaicing smaller images together. The software calculates how many smaller images are needed based upon the field of view at the magnification used, drives to the center of each rectangle, and then seemlessly stitches the images into one whole. The false colored BSE image of a cm-sized zoned garnet to the right was made by many (>100) 63x scans (each scan 1.9 mm max width). From research of Cory Clechenko and John Valley.