MEMS, NEMS and Contamination at the Atomic Scale: Part III
Barbara Kanegsberg & Ed Kanegsberg, November 2004
We conclude our discussion of NEMS and MEMS with a potpourri of techniques
including an additional MEMS based sensing tool, a modeling technique for predicting
the effects of contamination in MEMS devices, approaches to avoiding contamination
in MEMS and NEMS devices, and applications of nano-technology approaches to
contamination minimization at the macroscopic level.
Shear force microscopy
As discussed last month, atomic force microscopy (AFM) sensors detect
surface defects including particulate contamination. However, microscopic
device manufacturers
face additional surface issues including friction between contacting or
near contacting parts of the device. While conventional AFM measures
small displacements
of the probe perpendicular to the surface, a somewhat similar device, known
as Shear Force Microscopy (SFM or ShFM), or sometimes referred to as Transverse
Dynamic Force Microscopy (TDFM) uses a cantilever probe moving parallel
to the surface. It is never in actual contact with the surface but
is affected
by the dissipative forces associated with friction.1 In a sense, SFM measures
the lubricity or “stickiness” of the surface. It is particularly
useful for the study of softer materials and also enables non-destructive,
high resolution topography imaging of delicate samples such as thin layers
of organic molecules.
Modeling effects of contamination
Suppose you are designing a microdevice. It would be very useful to predict
the effects of contamination or defects on performance. The proposed
device design might be modified to minimize such effects. Researchers
at the Carnegie-Mellon
University have developed a computer modeling tool called CARAMEL (Contamination
And Reliability Analysis of MicroElectromechanical Layout) which is being
used to analyze the predicted impact of contamination particles on the
structural and material properties of MEMS.2 Given MEMS design, a particulate
description,
and a process (fabrication) recipe, CARAMEL performs a process simulation
and
creates a three-dimensional representation of the resulting defective
structure. The CARAMEL output can then be used as part of the input to
the Finite Element
Analysis (FEA), a computerized analysis tool that analyzes how various
parts of a structure will interact, including expected stresses and strains.
Thus
CARAMEL allows designers to identify specific potential areas of vulnerability
without incurring the costs of prototype development.
Preventing contamination in MEMS and NEMS devices
MEMS or NEMS devices are especially vulnerable to damage during latter
stages of fabrication. It is not uncommon for the back end of the
MEMS product flow
to represent 80% or more of the total cost.3 The presence of mechanical
structures on the surface adds fragility. For example, many MEMS
devices are produced
on a wafer; and pre-packaging steps, such as wafer dicing, may introduce
contamination or damage. Surface contamination can lead to impaired
performance.
A number of techniques have been developed to protect micro-mechanical
devices. Encapsulation techniques include a “Near” hermetic
package of molded liquid crystal polymer, designed to provide
protection but at a reasonable
cost.
A second technique has been developed to protect MEMS vacuum
devices. Macroscopic vacuum devices frequently use getters
to maintain high
vacuum conditions.
Getters contain molecules with a high affinity for gaseous
contaminants, sweeping them
out of the cavity (a bit like flypaper). Unfortunately, traditional
sintered metal getters can themselves be a major source of
particulate contamination
that can migrate into the mechanical regions of the MEMS
device.5 A thin-film getter known as NanoGetter™ is a more
appropriate solution for MEMS vacuum devices by providing the gettering
function
without
particle generation. Nano-technology
approaches to preventing contamination of macroscopic structures.
Nano-technology approaches to preventing contamination
of macroscopic structures
Sometimes the best protection from contaminates are other
contaminates. Two nano-technology techniques are being
commercially developed
with potential
application to consumer products. Nano-structures have
been developed that mimic the hydrophobic (water repelling)
properties
of the lotus
plant.6 The
precursors of these structures are supplied as an aerosol
spray coating containing hydrophobic polymers such as
polypropylene, polyethylene,
and waxes. As it
dries, the coating forms a nanostructure through self-assembly.
Al-though current target applications are consumer products
including paper
and leather, similar
techniques could be developed to protect microscopic
devices or for other fabrication applications.
A somewhat similar approach is employed for “self-cleaning” glass.7
An extremely thin (~40nm) coating of titanium dioxide
provides both a hydrophilic property as well as a catalytic
property
that causes
solar UV light to
break down organic contaminants on the surface. The
surface has an exceedingly high
contact angle, on the order of 140 degrees. The surface
is effectively soiled with a specific contaminate.
As a result, water sheets
right off, carrying
dirt particles with it.
References:
1 K. Karrai, I. Tiemann. Phys. Rev. B, Vol. 62, No.
19, (2000) p. 13174.
2 A. Kolpekwar, T. Jiang, R. D. Blanton' Journal
Of Microelectromechanical Systems, Vol. 8, No.
3, (1999) p. 309.
3 R. Markunas. A2C2 Magazine, (February, 2003).
4 Flipchips.com, tutorial #36.
5 D. Sparks, S. Massoud-Ansari, N. Najafi. IEEE
Transactions on Advanced Packaging, Vol. 26, No.
3, (August 2003).
6 Nanotechweb.org, 11/8/02 BASF Lotus-Effect™.
7 Nanotechweb.org, (09/19/02).
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.
