MEMS, NEMS and Contamination at the Atomic Scale, Part I
Barbara Kanegsberg and Ed Kanegsberg, September 2004
As electronic and mechanical sensors become microscopic, our world-view of
contamination issues has to adjust. A few years ago, MEMS was a catchword.
MEMS, or Micro-Electro-Mechanical Systems, combines mechanical elements, sensors,
actuators, and electronics on a common silicon substrate. The technology is
already used for pressure sensors, accelerometers and more. In a recent column,
we discussed a SAW contamination surface film detector based on MEMS technology.1
More recently, the term NEMS has been introduced, with the îMicroî becoming ìNano.î These smaller devices have the promise of even greater sensitivity, and much lower power consumption. As detection sensitivity increases, an array of new or rarely used words are likely to become commonplace. Milli-, micro-, and nano- prefixes are being joined by atto- (10-18), zepto- (10-21), and yocto- (10-24›. NEMS-based mass sensors have been proposed as part of a toolkit to be used for the inexpensive real-time analysis or detection of toxics or explosives. Protein molecules or viruses could be distinguished from one another by weight or by the affinity for a particular surface (e.g. body-antibody).2
Nanotribology takes the study of friction and adhesion and lubrication
down to the molecular or atomic level. New contamination sensors,
which are sensitive
to extremely small quantities (including a balance with potential to weigh
a single atom!), are under development. We will discuss some of these new
sensors in greater detail next month.
An inherent consequence of the expected trend toward NEMS-based devices
will be increased significance of surface attributes including cleanliness,
potential
contamination sources, and surface quality or surface attributes. As
dimensions decrease, the surface-to-volume ratio increases. At the
ultimate limit
one could envision a situation where the entire device is a monolayer
surface with
an analogous power and delicacy that biologists recognize in membrane
structures.
In such situations, surface contamination is an inherently important
issue. Along the same line of thought, contamination particles that
may be insignificant
specks on a larger device may be virtual boulders on a NEMS device.
Microscopy reveals that even the smoothest surface has roughness
or structure at
high magnification. Both MEMS and NEMS involve both structural and
mechanical aspects, either rotating parts or vibrating parts. Contaminating
particles
can interfere
with mechanical motion either by physical constraint (a microscopic ìshoe
in the doorî) or by changing balance conditions and degrading (or shifting)
vibration resonances.
Another consequence of the higher surface-to-volume ratio is that the
device physics is more dominated by surface effects than bulk
effects. NEMS devices
may have a substantial fraction of the total atoms at or near
the surface.3 At this point, mechanical aspects such as tensile strength
change from
bulk effects to surface effects. Particles or thin films, such
as deposited from
airborne molecular contamination,4 can interfere with surface
physics
effects. MEMS or NEMS developers face some of the same cleanliness
issues as those
in wafer fabrication, with the added complication that most of
the mechanical devices are not planar, making it harder to reach
all
surfaces. Since
in some
cases it has been demonstrated that supposed surface contamination
can be actually a positive surface attribute,5 one can envision
the possibility
of ìcontaminationî used
as a protective measure on microscopic devices.
Note: Thanks to Dr. Panos Datskos, Research Staff Member at Oak Ridge
National Laboratory and a Research Associate Professor at the University
of Tennessee,
for his helpful contributions to and review of this column.
References
1 B. Kanegsberg, E. Kanegsberg. ìContamination in and out of the Cleanroom,î A2C2,
(November, 2003).
2 N. Lavrik, P. Datskos. ìNano-mechanics Weighs In,î Physics
World, (April 2004); available at http://physicsweb.org/article/world/17/4/3.
3 M. L. Roukes. ìNanoelectromechanical systems face the future,î Physics
World, (February, 2001); available at http://physicsweb.org/article/world/14/2/8.
4 B. Kanegsberg, M. Chawla. A2C2, op. cit. (February-June, 2001).
5 B. Kanegsberg, M. Chawla. A2C2, op. cit. (January-February,
2002).
Barbara Kanegsberg and Ed Kanegsberg
are independent consultants in critical cleaning, precision cleaning, surface
preparation, and contamination control. They are the editors of “Handbook
for Critical Cleaning,” CRC Press. Contact them at BFK Solutions
LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
