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We present a comparison of different methods to extract area information from images. Two different physical-based
algorithms were tested which determine the areas of arbitrarily shaped 3D nano-structures on wafers or photo-masks
(e.g. contact holes) using secondary electron images of scanning electron microscopy (SEM). One of these algorithms,
called NANOAREA, was developed by the PTB. The other one is the software package MaskEXPRESS, which was
developed by Toppan Printing Co., Ltd.
In addition to real SEM images we used Monte Carlo generated SEM images of contact holes of different shapes and
sizes. For this, the Monte Carlo simulation program MCSEM, developed at PTB, was applied. MCSEM simulates the
electron diffusion and secondary electron generation and transport in solid state material and provides simulated SEM
images of arbitrary 3D specimen structures.
NANOAREA uses basic image processing routines to estimate the edge position of a structure. Then, one-dimensional
profiles which intersect the structure boundary perpendicularly are extracted. A one-dimensional edge detection
algorithm determines the edge position on each profile. Finally these detected edge positions are used to calculate
the polygon area using the triangle method. NANOAREA showed a very small underestimation of the area of about
0.3 % with regard to the Monte Carlo simulations (i.e. sub-pixel deviation).
MaskEXPRESS has a similar approach, however employs a different edge detection algorithm. For quadratic contact
holes a very high correlation coefficient r larger than 0.99 of the CDs was seen with an offset of about 0.3 nm for the
two tested programs. Here the critical dimension (CD) is defined as the square root of the area. The deviations from
the mean offset were smaller than 1 nm over the whole investigated range. For analysis of arbitrarily shaped features
we used a double T-shaped structure. Also here almost perfect correlation was found (r = 0.98). The observed mean
offset in this case was also about 0.3 nm. The offsets depend on the length of the edge and can vary with the shape of
the structure, too.
Here we report the excellent correlation of the investigated algorithms and programs to determine area parameters
from SEM images. The results found are an important prerequisite for harmonized area measurement based on
independent algorithms and pave the way to a standardized approach to area determination and reporting of
photomask structures.
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