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

摘要

Patterned Si surfaces, p- and n-type doped, were examined for different secondary electron yield (contrast between p-type 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.