Page 16 - Principles and Applications of NanoMEMS Physics

2 Chapter 1

chip doubles every 18 months [2]. The culmination of such miniaturization
program, it is widely believed, is the demise of Moore’s law, whose
manifestation is already becoming apparent due to an increasing
predominance of the quantum mechanical nature of electrons in determining
the behaviour of devices with critical dimensions (roughly) below 100 nm.
This line of development is closely related to the field of quantum
devices/nanoelectronics, which was prompted by the conception of a number
of atomic-level deposition and manipulation techniques, in particular,
molecular beam epitaxy (MBE), originally exploited to construct laboratory
devices in which the physics of electrons might be probed and explored,
following the discovery of electron tunnelling in heavily-doped pn-junctions
[3]. Nanoelectronics did produce interesting physics, for instance, the
discovery of Coulomb blockade phenomena in single-electron transistors,
which manifested the particle nature of electrons, and resonant tunnelling
and conductance quantization in resonant tunnelling diodes and quantum
point contacts, respectively, which manifested the wave nature of electrons
[4-6]. These quantum devices, in conjunction with many others based on
exploiting quantum phenomena, generated a lot excitement during the late
1980s and early 1990s, as they promised to be the genesis for a new digital
electronics exhibiting the properties of ultra-high speed and ultra-low power
consumption [7-8]. While efforts to realize these devices helped develop the
skills for fabricating nanoscale devices, and efforts to analyze and model
these devices helped to develop and mature the field of mesoscopic quantum
transport, the sober reality that cryogenic temperatures would be necessary
to enable their operation drastically restricted their commercial importance.
A few practical devices, however, did exert commercial impact, although
none as much as that exerted by silicon IC technology, in particular,
heterojunction bipolar transistors (HBTs), and high-electron mobility
transistors (HEMTs), which exploit the conduction band discontinuities
germane to heterostructures, and modulation doping to create 2-D electron
confinement and quantization, respectively, and render devices superior to
their silicon counterparts for GHz-frequency microwave and low-transistor-
count digital circuit applications [9-14].
The commercial success of the semiconductor industry, and its
downscaling program, motivated emulation efforts in other disciplines, in
particular, those of optics, fluidics and mechanics, where it was soon
realized that, since ICs were fundamentally two-dimensional entities,
techniques had to be developed to shape the third dimension, necessary to
create mechanical devices exhibiting motion and produced in a batch planar
process [15]. These techniques, which included surface micromachining,
bulk micromachining, and wafer bonding, became the source of what are
now mature devices, such as accelerometers, used in automobile air bags,