Figure A.2:
Differently shaped trenches with their characteristic dimensions
which are used to determine the aspect ration (AR).
The aspect ratio (AR) of a trench is defined as the ratio between the height
and the width or length
of a trench and is used as a characteristic
quantity especially for chemical processes like material etching and depositing.
For chemical material deposition, the aspect ratio calculation for rectangular
trenches has been introduced in [332] (cf. Figure A.2a) using
(A.12)
However, fabrication processes in general do not provide rectangular-shaped
trenches because etching processes always produce tilted side walls and in the entry
section of the trench at the top there is often an additional rounding
according to surface diffusion during material deposition.
Nevertheless, the impact of the tilted side walls on the width and the height
of the particular trench can often be neglected with respect to the overall dimensions
of the trench. Then, a rectangular shape can be justified.
However, if the trench can not sufficiently be approximated by a rectangle, a
different metric has to be applied to provide a characteristic criterion to
compare different trenches using a different calculation of AR
(A.13)
where the height and width are approximated by the mean value
and
, respectively.
For instance, the height and the width can be approximated by their mean values
as