We have performed high resolution transmission electron microscope
(HRTEM) image simulations to qualitatively assess the visibility
of various structural defects in ultrathin gate oxides of MOSFET
devices, and to quantitatively examine the accuracy of HRTEM
in performing gate oxide metrology. Structural models contained
crystalline defects embedded in an amorphous 16-Å-thick
gate oxide. Simulated images were calculated for structures
viewed in cross section. Defect visibility was assessed as a
function of specimen thickness and defect morphology, composition,
size, and orientation. Defect morphologies included asperities
lying on the substrate surface, as well as “bridging”
defects connecting the substrate to the gate electrode.
Measurements of gate oxide thickness extracted from simulated
images were compared to actual dimensions in the model structure
to assess TEM accuracy for metrology. The effects of specimen
tilt, specimen thickness, objective lens defocus, and coefficient
of spherical aberration (Cs) on measurement accuracy
were explored for nominal 10-Å gate oxide thickness. Results
from this work suggest that accurate metrology of ultrathin
gate oxides (i.e., limited to several percent error) is feasible
on a consistent basis only by using a Cs-corrected
microscope. However, fundamental limitations remain for
characterizing defects in gate oxides using HRTEM, even with
the new generation of Cs-corrected microscopes.
HRTEM image simulations of structural defects in gate oxides
