29-05-2012, 12:54 PM
NANOELECTROMECHANICAL SYSTEMS
NANOELECTROMECHANICAL SYSTEMS.pdf (Size: 616.01 KB / Downloads: 84)
ABSTRACT
Nanoelectromechanical systems, or NEMS, are MEMS scaled
to submicron dimensions [1]. In this size regime, it is possible to
attain extremely high fundamental frequencies while simultaneously
preserving very high mechanical responsivity (small force
constants). This powerful combination of attributes translates
directly into high force sensitivity, operability at ultralow power,
and the ability to induce usable nonlinearity with quite modest
control forces. In this overview I shall provide an introduction to
NEMS and will outline several of their exciting initial applications.
However, a stiff entry fee exists at the threshold to this new
domain: new engineering is crucial to realizing the full potential of
NEMS. Certain mainstays in the methodology of MEMS will,
simply, not scale usefully into the regime of NEMS.
INTRODUCTION
NEMS have a host of intriguing attributes. They offer access
to fundamental frequencies in the microwave range; Q’s, i.e.
mechanical quality factors, in the tens of thousands (and quite
possibly much higher); active masses in the femtogram range; force
sensitivities at the attonewton level; mass sensitivity at the level of
individual molecules, heat capacities far below a “yoctocalorie” [2]
— this list goes on. These attributes spark the imagination, and a
flood of ideas for new experiments and applications ensues.
MULTITERMINAL MECHANICAL DEVICES
The attributes of NEMS described in the next section make
clear that we should be envisioning applications for
electromechanical devices with response times and operating
frequencies that are as fast as most of today’s electron devices.
Furthermore, multiterminal electromechanical devices are possible
F i.e. two-, three-, four-ports, etc. F LQ ZKLFK HOHFWURPHFKDQLFDO
transducers provide input stimuli (i.e. signal forces), and read out a
mechanical response (i.e. output displacement). At additional
control terminals, electrical signals either quasi-static or timevarying
can be applied, and subsequently converted by the
control transducers into quasi-static or time-varying forces to
perturb the properties of the mechanical element in a controlled,
useful manner. The generic picture of this scheme is shown in
Figure 1
NEMS ATTRIBUTES
Frequency. Table 1 displays attainable frequencies for the
fundamental flexural modes of thin beams, for dimensions spanning
the domain from MEMS (leftmost entries) to deep within NEMS.
The mode shapes, and hence the force constants and resulting
frequencies, depend upon the way the beams are clamped; Table 1
lists the results for the simplest, representative boundary conditions
along three separate rows. The last column represents dimensions
currently attainable with advanced electron beam lithography. Of
course, even smaller sizes than this will ultimately become feasible;
clearly the ultimate limits are reached only at the molecular scale.
Nanodevices in this ultimate limit will have resonant frequencies in
the THz range, i.e. that characteristic of molecular vibrations.
Each entry is in three parts, corresponding to structures made
from silicon carbide, silicon, and gallium arsenide. These materials
are of particular interest to my group, and are among the
“standards” within MEMS. They are materials available with
extremely high purity, as monocrystalline layers in epitaxially grown
heterostructures. This latter aspect yields dimensional control in
the “vertical” (out of plane) dimension at the monolayer level. This
is nicely compatible with the lateral dimensional precision of
electron beam lithography that approaches the atomic scale. The
numbers should be considered loosely as “typical”; they represent
rough averages for the various commonly used crystallographic
orientations.
CONCLUSIONS
NEMS offer access to a parameter space for sensing and
fundamental measurements that is unprecedented and intriguing.
Taking full advantage of it will stretch our collective imagination, as
well as our current methods and “mindsets” in micro- and
nanodevice science and technology. It seems certain that many new
applications will emerge from this new field. Ultimately, the
nanomechanical systems outlined here will yield to true
nanotechnology. By the latter I envisage reproducible techniques
allowing mass-production of devices of arbitrary complexity, that
comprise, say, a few million atoms í each of which is placed with
atomic precision [38]. Clearly, realizing the “Feynmanesque”
dream will take much sustained effort in a host of laboratories.
Meanwhile, NEMS, as outlined here, can today provide the crucial
scientific and engineering foundation that will underlie this future
nanotechnology.
NANOELECTROMECHANICAL SYSTEMS.pdf (Size: 616.01 KB / Downloads: 84)
ABSTRACT
Nanoelectromechanical systems, or NEMS, are MEMS scaled
to submicron dimensions [1]. In this size regime, it is possible to
attain extremely high fundamental frequencies while simultaneously
preserving very high mechanical responsivity (small force
constants). This powerful combination of attributes translates
directly into high force sensitivity, operability at ultralow power,
and the ability to induce usable nonlinearity with quite modest
control forces. In this overview I shall provide an introduction to
NEMS and will outline several of their exciting initial applications.
However, a stiff entry fee exists at the threshold to this new
domain: new engineering is crucial to realizing the full potential of
NEMS. Certain mainstays in the methodology of MEMS will,
simply, not scale usefully into the regime of NEMS.
INTRODUCTION
NEMS have a host of intriguing attributes. They offer access
to fundamental frequencies in the microwave range; Q’s, i.e.
mechanical quality factors, in the tens of thousands (and quite
possibly much higher); active masses in the femtogram range; force
sensitivities at the attonewton level; mass sensitivity at the level of
individual molecules, heat capacities far below a “yoctocalorie” [2]
— this list goes on. These attributes spark the imagination, and a
flood of ideas for new experiments and applications ensues.
MULTITERMINAL MECHANICAL DEVICES
The attributes of NEMS described in the next section make
clear that we should be envisioning applications for
electromechanical devices with response times and operating
frequencies that are as fast as most of today’s electron devices.
Furthermore, multiterminal electromechanical devices are possible
F i.e. two-, three-, four-ports, etc. F LQ ZKLFK HOHFWURPHFKDQLFDO
transducers provide input stimuli (i.e. signal forces), and read out a
mechanical response (i.e. output displacement). At additional
control terminals, electrical signals either quasi-static or timevarying
can be applied, and subsequently converted by the
control transducers into quasi-static or time-varying forces to
perturb the properties of the mechanical element in a controlled,
useful manner. The generic picture of this scheme is shown in
Figure 1
NEMS ATTRIBUTES
Frequency. Table 1 displays attainable frequencies for the
fundamental flexural modes of thin beams, for dimensions spanning
the domain from MEMS (leftmost entries) to deep within NEMS.
The mode shapes, and hence the force constants and resulting
frequencies, depend upon the way the beams are clamped; Table 1
lists the results for the simplest, representative boundary conditions
along three separate rows. The last column represents dimensions
currently attainable with advanced electron beam lithography. Of
course, even smaller sizes than this will ultimately become feasible;
clearly the ultimate limits are reached only at the molecular scale.
Nanodevices in this ultimate limit will have resonant frequencies in
the THz range, i.e. that characteristic of molecular vibrations.
Each entry is in three parts, corresponding to structures made
from silicon carbide, silicon, and gallium arsenide. These materials
are of particular interest to my group, and are among the
“standards” within MEMS. They are materials available with
extremely high purity, as monocrystalline layers in epitaxially grown
heterostructures. This latter aspect yields dimensional control in
the “vertical” (out of plane) dimension at the monolayer level. This
is nicely compatible with the lateral dimensional precision of
electron beam lithography that approaches the atomic scale. The
numbers should be considered loosely as “typical”; they represent
rough averages for the various commonly used crystallographic
orientations.
CONCLUSIONS
NEMS offer access to a parameter space for sensing and
fundamental measurements that is unprecedented and intriguing.
Taking full advantage of it will stretch our collective imagination, as
well as our current methods and “mindsets” in micro- and
nanodevice science and technology. It seems certain that many new
applications will emerge from this new field. Ultimately, the
nanomechanical systems outlined here will yield to true
nanotechnology. By the latter I envisage reproducible techniques
allowing mass-production of devices of arbitrary complexity, that
comprise, say, a few million atoms í each of which is placed with
atomic precision [38]. Clearly, realizing the “Feynmanesque”
dream will take much sustained effort in a host of laboratories.
Meanwhile, NEMS, as outlined here, can today provide the crucial
scientific and engineering foundation that will underlie this future
nanotechnology.