Thursday, June 16, 2011

Charging & Low Voltage SEM

When insulating samples are imaged at high beam energies in the SEM they are subject to charging. Samples don't charge when the current into the sample as incident beam current (I0) equals the current leaving the sample as secondary electrons (ISE) and backscattered electrons (IBE):

I0 = ISE + IBE

At high beam energies secondary production is very efficient due to an increased number of energy loss channels, and thus samples tend towards a positive charge. The result is a slow and steady increase of charge at the sample surface resulting in electrostatic fields that deflect the incident beam and the trajectories of secondary electrons and backscattered electrons leaving the sample. In an SEM this manifests as time varying changes in contrast and brightness and sudden changes in the image position or image quality. This image of uncoated cellulose taken at 20 kV shows these signs of charging. At the top right portion of the image one can see a cellulose fiber that has been physically deflected by electrostatic charge during imaging. Note the very bright "flaring" portions of the image.

By imaging at very low beam energies one can generally find a beam energy where there is no charging. Because of this the JEOL 5900 low voltage SEM (LV-SEM) can adjust its beam energy in 100 V steps from 300 V to 2500 V. Finding the ideal beam energy to avoid charging can take some finessing, but 1500 to 2000 V is a good starting point. One should keep in mind that if one does manage to induce charging in the sample with too high of a beam voltage that this charging will only very slowly dissipate over time. As such, try optimizing the beam voltage with insulating samples on an edge of the sample. If one charges this area, one can move to another part of the sample margin and so on until one has optimized the beam voltage. Then one can move to the center of the sample and image it without sample charging and without coating.

This is an image of teflon tape (the type used in plumbing) imaged at a beam voltage of 2500 V without coating. Note the absence of charging and the presence of domains connected by fibers. While the ability to image at low voltages without sample coating is nice, it does provide decreased resolution in the JEOL 5900. As the beam voltage decreases the effects of astigmatism are greatly enhanced. Imaging at low voltages will require not only readjustment of stigmators but ideally a column realignment.

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