'Nanotractor' studies micro-scale friction
Interest in the development of MEMS
(microelectromechanical systems) has
grown steadily during the past decade.
These tiny devices, now used in such
applications as auto airbag systems, inkjet
printers, and display units, are attractive
because they take up little space and
require little or no assembly. They also are
cheap to produce in batch quantities
because they are made with a technology
that is already mature -- the microlithography
used to make silicon chips.
Arguably, MEMS could prove to be a
ubiquitous "disruptive technology" that
transforms a variety of engineering
disciplines just as the silicon chip
transformed electronics engineering. Yet
the process of integrating MEMS into new
systems has been slow. An overarching
reason is that physics -- especially the
effects of adhesion and friction --
sometimes works differently at the microscale.
Some assumptions seem intuitively
correct because they apply to conventional
machines. For example, Amontons' law,
first stated 300 years ago, says friction is
proportional to force applied normal to
(perpendicular to) a surface. Yet relying
on this assertion could prove disastrous
when applied to devices the size of a
At Sandia, where engineers are eager
to pack increased capability into very
small spaces, the development of tools to
manipulate microdevices -- one example
is a microtweezers -- has been in high
gear for over a decade. More recently,
research has turned to the use of nanotechnology
as an "enabling technology"
to make better MEMS devices.
Friction or "stiction" -- the tendency
of small parts to adhere to one another --
has proven a significant hindrance to
MEMS development. Treating the
touching or sliding surfaces of a
micromachine with a slippery molecular
monolayer can reduce friction. However,
there remains the need for a tool to
measure the friction between two MEMS
surfaces accurately to determine exactly
what conditions are most effective in
reducing it. With this information in hand,
engineers will gain an extremely
important tool in modeling MEMS
devices prior to their actual manufacture.
But creating a device small enough to
measure friction on a MEMS device is no
easy task, for the tool must be about the
width of a human hair.
In order to study friction at the microscale,
Sandia's Maarten de Boer and
coworkers set about building a polysilicon
actuator that would controllably and
accurately generate both very low and
very high forces, and apply them both
perpendicularly and tangentially.
The resulting "nanotractor" design
incorporates an actuation plate in its
central section and frictional clamps on its
two ends. In the clamps, load is applied
electrostatically but borne mechanically to
develop friction forces. To obtain motion,
the leading clamp is fixed in place with a
large voltage. The plate is then actuated
by attracting it toward the substrate.
Because the actuation plate is now
bending, the trailing clamp, which is not
loaded, slides a short distance (about 40
nanometers) toward the leading clamp.
The trailing clamp is now held fixed with
a large voltage, and the voltages on the
leading clamp and plate are turned off.
The leading clamp then slips forward.
This stepping cycle is applied repeatedly
to obtain large-scale motion with very
De Boer and postdoctoral researcher
Alex Corwin determined that this
nanotractor operates at up to 80,000
cycles a second, with a velocity of up to 3
millimeters per second. A maximum force
of 2.5 millinewtons is achieved when the
nanotractor stalls out, about 250 times
more force than a comb drive.
Defying the law
Working with Corwin, de Boer found
that the coefficient of static friction began
to increase at low normal loads (below 50
micronewtons). De Boer attributes this
deviation from Amontons' law to adhesive
forces. They also observed sliding of up
to 200 nanometers before the static friction
event. This means Amontons' law is also
not valid over short sliding distances.
Besides serving as a test structure for
model friction studies, the nanotractor
actuator is attractive for other uses. Marc
Polosky, a Sandia staff member, has
demonstrated its use in a MEMS system
that performs mechanical logic functions.
It also may prove useful for the precise
positioning and control of micro-optical
"Modelers are excited by the
nanotractor test results, and now face the
challenge of understanding and modeling
newly observed phenomena such as gross
slip prior to sliding," says Sandia staff
member Dave Reedy. "Our goal is to
develop a capability to perform
simulations of MEMS components that
accurately predict response in the
presence of adhesion and friction."
Technical Contact: Maarten de Boer
Media Contact: Michael Padilla