Thursday, June 16, 2011

Directionality of SEM Imaging Signals

When performing SEM imaging it is important to have an intuitive sense of the directionality-- or lack thereof-- of the signals being used. All SEM images are maps of position, which is generated by the rastering of the SEM beam by the scan coils, and some signal related to the interaction of the electron beam in the sample. The most common of these signals is secondary electrons which are detected by the Everhart-Thornley or "ET" detector. The ET-detector uses a grid biased at positive voltage to suck the low energy secondary electrons towards a scintillator where they are converted into light and then detected by a photomultiplier. The secondary electron detection efficiency of the ET-detector is very high, allowing it to detect secondary electrons emitted from virtually every portion of complex surface morphology hit by the primary beam. While there may be some changes in detection efficiency with topography, there is never any portion of the structure where the ET-detector is masked or blocked. This image of a TEM grid is a good example. Even though the ET-detector is oriented towards the top of the image, the topographical contrast of the grid structure does not mask the detection of the secondary electrons. The brighter than expected edges are also a signature of the excellent detection efficiency of the ET-detector. These are edge effects due to the ET-detector being able to collect secondaries that are emitted not only at the top of the sample, but at the edges of the sample.

In this X-ray map of the Cu Ka line, it is immediately obvious that the TEM grid morphology is masking the Cu Ka X-rays generated by the primary electron beam. The EDS detector comes in near the top left quadrant of the image-- masking is absent in the top left quadrant of the TEM grid and present in each of the three other quadrants. Only X-rays generated in direct line of sight to the EDS detector are visible without attenuation. This is of concern in the interpretation of EDS X-ray maps, but also in the EDS point analysis on samples with non-flat morphology.

No comments:

Post a Comment