Device Metrology with high-performance scanning ion beams

A scanning ion microscope (SIM) is analogous to a scanning electron microscope (SEM) but utilizes a beam of helium ions, with energy of 10 to 25 keV, instead of electrons. The SIM potentially offers several advantages for device critical dimension metrology as compared to the more familiar CD-SEM. These include a high brightness source which is sub-nanometer in size, an enhanced secondary electron yield, restricted beam penetration, and superior image contrast and information content. Possible problems include pervasive positive charging, ion implantation, and a lack of detailed experimental and theoretical knowledge about low energy ion interactions with solids. Comparison of line profiles across structures made by electron induced and ion induced secondary electrons show that there are some significant differences between them which arise from the different modes of interaction in the two cases. As a result the algorithms employed for line width determination Will require revision in order to produce data which is consistent with CD-SEM data.