Dr. Christopher G. Walker Profile

Dr. Christopher G. Walker

at Univ of York

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Proceedings Article | 22 May 2009 Paper

The role of oxygen in secondary electron contrast of doped semiconductors in LVSEM

M. El Gomati, F. Zaggout, C. Walker, X. Zha

Proceedings Volume 7378, 73780Y (2009) https://doi.org/10.1117/12.821810

KEYWORDS: Scanning electron microscopy, Electron beams, Semiconductors, Oxides, Silicon, Carbon, Oxygen, Ions, Molybdenum, Xenon

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Patterned Si surfaces, p- and n-type doped, were examined for different secondary electron yield (contrast between ptype and n-type regions) under the electron beam of a scanning electron microscope. The contrast as a function of primary beam energy was studied for samples with a thick oxide layer and with the layer removed using an HF solution. It was found that the contrast between p- and n- type areas reversed on the samples with a thick oxide layer as the primary beam energy was increased. However, after the oxide layer was removed, the contrast reversal was no longer apparent. In addition, it was also found that regions on a patterned Si sample could reverse in contrast when the scan speed of the electron beam was changed. The various competing theories describing the dopant contrast effect of doped semiconductors are discussed and compared to the results reported here and elsewhere in the literature. It is concluded that oxygen at sub-monolayer coverage through to thick films plays an important role in the dopant contrast effect. However, adventitious carbon is equally important where a metal-oxide-semiconductor structure could exist with the presence of these two materials. Results from the literature using other techniques such as photoemission and field emission are also considered and it is found that these studies give results which are inconsistent with several of the current theories which attempt to explain the dopant contrast effect.

Proceedings Article | 22 May 2009 Paper

Recent developments in the understanding and application of backscattered and secondary electrons in the SEM

M. M. El Gomati, C. G. H. Walker, J. A. D. Matthew

Proceedings Volume 7378, 73780Z (2009) https://doi.org/10.1117/12.821809

KEYWORDS: Electrons, Scattering, Transition metals, Copper, Metals, Ions, Scanning electron microscopy, Monte Carlo methods, Argon, Electron transport

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The secondary electron and backscattered electron coefficients have been measured as a function of primary beam energy for as-inserted and cleaned pure element samples. Clearly, the effect of cleaning samples makes a significant effect on both these key measurements needed for understanding the electron transport measurements in scannng electron microscopy and a number of other technologies. The results from the cleaned samples suggest that the currently accepted theory for secondary electron emission (SEE) of Baroody does not take account of an important physical effect. We propose that the SEE in transition metals is mainly controlled by the inelastic mean free path (IMFP) of the secondary electrons. In combination with current theories on the transport of hot electrons in transition metals, where sensitivity to the density of empty d states is important, the apparent correlation of the work function with SEE can be explained. The effect of errors in the electron elastic scattering cross-section and the electron stopping power on the estimates of backscattered electron coefficient, η, are explored for the case of Cu. It is found that percentage errors in one parameter (e.g. stopping power) cause very similar changes in η as equal but opposite percentage errors in the other parameter (e.g. elastic scattering cross-section).

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