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Motivation (Page Best Viewd in Inetrnet Explorer)
The motivation for
this work lies in the important role played by macromolecules, such as
surfactants and alkylsilanes in the fabrication of microelectromechanical
structures (MEMS). MEMS are miniaturized mechanical components such as
membranes, motors, gears, pumps, and valves, which are fabricated using
surface micromachining techniques. Such microstructures have lateral
dimensions 50-500 microns, with thickness 0.1-2.5 microns, and are
offset 0.1-2 microns from the substrate. The basic steps in the surface
micromachining are illustrated in the Figure 1 below. First the
substrate is typically coated with an isolation layer that
protects it during the subsequent etching steps. A sacrificial layer is
then deposited on the substrate and patterned. The microstructure thin
film is then deposited and etched. Finally, selective etching of the
sacrificial layer creates the free standing micro mechanical structures
such as the cantilever beam shown in the Fig.
Given the large
surface area to volume ratio of the microstructures, they are
particularly vulnerable to sticking to the substrate or adjacent
microstructures during release or later, during use. This phenomenon is
more generally called stiction. Deposition of self-assembled hydrophobic
monolayers on the hydrophilic surfaces of MEMS is one of the most
successful approaches for minimizing stiction in these devices.
Traditionally, such hydrophobic coatings on the hydrophilic MEMS
substrate have been deposited from organic solutions. However, because
of the
increasing concerns regarding organic wastes in the work places, latest research efforts
have focused on developing chemistries that would allow
deposition of hydrophobic coatings from aqueous solutions.
Besides
stiction, friction is a major concern in the operation of those MEMS
devices that have components (motors, gears, etc.) with moving
surfaces in contact. These surfaces undergo friction resulting in
wear debris detrimental to the operation of MEMS devices. In order
to minimize such losses, duplex wear resistant and anti-stiction
coatings have been proposed in which a tungsten oxide coating is
first applied to the exposed surfaces from an aqueous based
precursor solution and then an alkylsilane coating is applied over
tungsten oxide. Tungsten coating minimizes friction losses because
of its superior wear properties where as silane coating provides
anti-stiction properties.
Because of
their role in the development of environmentally acceptable
methodologies, aqueous surfactant and silane solutions are systems
of extreme technological importance. The coatings deposited on the
MEMS systems are roughly a monolayer (or a few monolayers) thick. It
is, therefore, imperative to understand the underlying physics and
chemistry of these systems at the molecular level for enhancements
in the coating methods and quality. However, existing experimental
methods and tools are still limited in their scope to investigate
the material systems in sufficient detail at the length and time
scales involved in the deposition of silane monolayers on MEMS
surfaces.
Atomistic
simulation techniques such as Monte Carlo (MC) and molecular
dynamics (MD) are now well-established tools in the studies of
materials at the atomic and molecular level. MD and MC methods find
their origin in classical statistical mechanics. Provided a model
for the interactions between the atomic constituents of some system
exists (for instance, in the form of interatomic or intermolecular
potentials which describe the energy of the system as a function of
its microscopic degrees of freedom), one can sample
deterministically (MD) or stochastically (MC) the microscopic states
of the system. The microscopic degrees of freedom usually consist of
the set of positions and momenta of the particles. The original
intent of MD and MC is, once equilibrium is achieved, to use the
concepts of temporal averaging (MD) or statistical averaging (MC)
over the sampled microscopic states to calculate the properties of a
macroscopic system. In addition to determination of equilibrium
properties, MD methods are also used to study non-equilibrium system
properties.
The length and
time scales involved in the molecular level investigation of
structure of alkylsilane monolayers and its evolution under sliding
friction conditions are relevant to molecular simulations.
Therefore, there is a significant motivation for studying these
systems via MC and MD simulations. This work has examined, via
molecular simulations, the structure of self-assembled cationic and
non-ionic silane films on a charged polysilicon substrate and effect
of sliding friction on these monolayers.
The details of
simulation models, methods and simulation results of this research can be obtained from
the following links:
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