KEYWORDS: Contrast transfer function, Scanning electron microscopy, Spatial frequencies, Image resolution, Signal processing, Microscopes, Ions, Electron microscopes, Metrology, Spatial resolution
Although Scanning Electron Microscopes (SEM) have improved greatly over the last decade the techniques usually
employed to measure their performance have not changed significantly in half a century. In particular, describing
the imaging performance of an SEM by a single number - its 'resolution' - provides no useful information about its
real world imaging capabilities nor about any of the factors that might limit that usefulness of the SEM for tasks
such as metrology. The Contrast Transfer Function (CTF) discussed here analyses the way in which the SEM
processes signal components of different spatial frequencies. The resultant plot provides information on the noise
limited spatial resolution limit, predicts how this will vary with noise level, and provides a powerful general
diagnostic capability. This type of measurement, which has become standard practice for transmission electron
microscopes, can be performed using the public domain software package IMAGE-J, is rapid, and requires only a
specimen offering a broad and flat Fourier spectrum. The capabilities of this approach are demonstrated by a
number of examples.
IONiSE is a Monte Carlo simulation which describes the interactions of 5-50 keV energy He+ ions with solids, and
predicts the production of ion induced secondary electron (iSE) emission. Its use to determine the most probable implant
depth, the maximum ion range, and the effect of straggle are presented. IONiSE has been used to numerically fit
literature tabulations of iSE generation from five elements so as to derive excitation energy and mean free path
parameters. By employing those parameters in IONiSE the topographic yield variation for iSE as a function of energy
and the atomic number of the target has been predicted, and estimates of the individual secondary electron contributions
from the incident and backscattered ions have been made. These simulations help to create a foundation for the
application and the interpretation of iSE images for metrology.
KEYWORDS: Ions, Ion beams, Metrology, Electron beams, Scanning electron microscopy, Helium, Image enhancement, Microscopes, Image resolution, Signal generators
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.
Conference Committee Involvement (3)
Scanning Microscopies 2012: Advanced Microscopy Technologies for Defense, Homeland Security, Forensic, Life, Environmental, and Industrial Sciences
24 April 2012 | Baltimore, Maryland, United States
Scanning Microscopies 2011: Advanced Microscopy Technologies for Defense, Homeland Security, Forensic, Life, Environmental, and Industrial Sciences
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