Friday, March 1, 2013

Tilt, Image Collection and Morphology


One of the keys to obtaining high quality SEM images is an understanding of image formation. A secondary electron image is a map of secondary electron intensity as a function of position. Where this becomes subtle is how are the secondary electrons detected?

In an Everhart-Thornley (ET) detector secondary electrons are collected by a slightly positively biased cage where they hit a scintillator and create light which is then subsequently detected using a photo-multiplier tube.

One trick in obtaining high quality images is learning to exploit the collection characteristics of the detectors one is using. In the case of the standard ET-detector, there is a sampling bias-- electrons emitted in the direction of the ET-detector are detected more efficiently. In the first image we see Au clusters of HOPG. The clusters are well resolved, but there is some hint of ordering of these clusters as indicated by the faint lines from the bottom left towards the top right of the image.

If we tilt the sample towards the ET-detector, then we will increase the secondary electron detection efficiency. In a sample like this there is very little vertical morphological contrast, so there is very little secondary electron collection contrast between higher and lower parts of the sample.

In the second image the sample is tilted + 30 degrees towards the ET detector. Now, even though it has vertical features of only a few 10's nm we can clearly see the structure of the cleaved surface of the HOPG. This level of structure is lost without the tilt of +30 degrees.

In the final image the sample is tilted - 10 degrees away from the ET detector. In this case all structure from the cleavage of the HOPG surface is lost. All we see in the final image is secondary electron contrast from the Au clusters, not from the terraces of the HOPG crystal.

The purpose of this short note is to show how to gain structural resolution by tilting the sample towards the ET-detector. Sample tilting is more than just seeing the undersides of things!

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