The effects of film thickness, electrode size
and substrate thickness on the impedance parameters of alternating frequency dielectric
measurements of insulating thin films deposited on conductive substrates were
studied through parametric finite-element simulations. The quasi-static forms
of Maxwell's electromagnetic equations in a time harmonic mode were solved
using COMSOL Multiphysics® for several types of 2D models (linear and
axisymmetric). The full 2D model deals with a configuration in which the
impedance is measured between two surface electrodes on top of a film deposited
on a conductive substrate. For the simplified 2D models, the conductive
substrate is ignored and the two electrodes are placed on the top and bottom of
the film. By comparing the full model and the simplified models, approximations
and generalizations are deduced. For highly insulating films, such as the case
of insulating SiO2 films on a conducting Si substrate, even the simplified
models predict accurate capacitance values at all frequencies. However, the
edge effects on the capacitance are found to be significant when the film
thickness increases and/or the top electrode contact size decreases. The
thickness of the substrate affects predominantly the resistive components of
the dielectric response while having no significant effect on the capacitive
components. Changing the electrode contact size or the film thickness
determines the specific values of the measured resistance or capacitance while
the material time constant remains the same, and thus this affects the
frequency dependence that is able to be detected. This work highlights the
importance of keeping in mind the film thickness and electrode contact size for
the correct interpretation of the measured dielectric properties of
micro/nanoscale structures that are often investigated using nanoscale
capacitance measurements.
Source:IOPscience
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