快猫短视频

THERE are many ways of bringing about change. Some take bloody acts of
violence. Others alter our lives through persuasive argument. Clark Nguyen would
also like to see our lives change, but he has his own way of achieving
it鈥攈e aims to do it without us even noticing. 鈥淭he way to make something
permeate society is to build it cheap enough and small enough so that it doesn鈥檛
intrude into everyday life,鈥� he says.

Nguyen, who is professor of electrical engineering and computer science at
the University of Michigan, hopes to revolutionise our lives by changing the way
that we communicate. In particular, he aims to shrink mobile phones and radios
until they鈥檙e small enough to mount inside wristwatches or, like the fictional
communicators of the most recent Star Trek movie, to be stuck as buttons
onto clothing.

Combining circuits for a transmitter and receiver鈥攁
transceiver鈥攐n one tiny device could let you do a lot more than simply
chat with faraway friends. Wear a transceiver on your shirt, says Nguyen, and
your house could recognise you, letting you in and out and automatically
adjusting the environment to suit your needs as you move from room to room. 鈥淚
see these things being literally everywhere,鈥� he says. 鈥淵ou could have a
television in your glasses, or something on your wrist that does
everything鈥攑hone, global positioning system, local area network.鈥�

Ever since the transistor was invented, researchers have been shrinking
individual electrical components. The big challenge is to integrate the parts to
avoid having to join them together with clumsy wires and connectors. The answer,
Nguyen believes, is to build them all on a single chip of silicon.

With this in mind, Nguyen has constructed a tiny version of a frequency
filter鈥攐ne of a radio鈥檚 key components鈥攖hat is only 21 micrometres
across. But it is not just the microscopic size of the device that is important.
Because Nguyen was able to build the filter using standard lithographic
techniques, it should be simple to construct the device as part of a silicon
chip.

Developing a tiny radio transceiver has proved remarkably difficult. Although
important components such as amplifiers have been miniaturised using integrated
circuit technology and built into silicon chips, it has been almost impossible
to do the same with other components. To solve the problem, Nguyen has chosen to
use tiny components that move鈥攎icroelectromechanical systems (MEMS).

Tiny moving machines

MEMS are minute devices that already perform many useful tasks鈥攁cting
as sensors to measure acceleration or as tiny actuators to control airflow
(鈥淚nvasion of the micromachines鈥�, 快猫短视频, 29 June 1996, p 28).
They are built up layer after layer by first transferring a pattern onto a
silicon wafer using lithography. Chemicals then etch away unexposed parts of the
silicon, and the process is repeated several times to produce a
three-dimensional structure. This is basically the same technique that
manufacturers use to construct the transistors and circuits of computer
chips.

The components of a radio that are perhaps the most difficult to transfer
onto a chip are the filters and oscillators on which they depend. Yet they are
essential. By resonating over a specific range of frequencies, they allow users
to tune in to a particular frequency while rejecting all the others. Most
receivers have a series of ever narrower filters. The final one lets past only a
single radio channel. Once through, the information carried within this channel,
be it music, speech or data, can be retrieved. 鈥淵our radio is just a frequency
selector,鈥� Nguyen says. 鈥淚ts main purpose is to grab just one of those radio
stations out there.鈥�

Existing filters tend to be based around quartz crystals, which oscillate at
a stable, precise frequency. The sharpness of the frequency response of an
oscillator or filter is measured by a property called its quality factor (Q),
which for quartz is between 10 000 and 100 000. By comparison, filters made from
discrete capacitors and inductors have a Q of only 50 or so. Worse, filters
etched in a silicon wafer can muster a Q of only about 10.

This is why Nguyen abandoned conventional electronics and opted for the
mechanical approach. This is yielding Qs of up to 100 000
(see Graph). 鈥淚f
you want to get all of the systems on a chip,鈥� he says, 鈥渢he only way to do it
is with MEMS technology.鈥�

Nguyen鈥檚 filter is made from a pair of parallel silicon beams, each mounted
on silicon anchors at both ends and free to vibrate in the middle. The beams are
joined by a thin, flexible piece of silicon. If one beam moves, the connecting
silicon 鈥渟pring鈥� transmits the motion to its partner
(see Diagram).

Just 100 nanometres beneath the centre of each beam is an electrode. An
incoming signal applied to one of the electrodes鈥攖he input
electrode鈥攇enerates a rapidly varying charge. Simple electrostatic forces
alternately attract, then repel, the beam above it. If the frequency of the
radio signal coincides with the resonant frequency of the beams, first one beam
and then the other starts to vibrate. The movement of the second beam generates
a varying electrical signal in the output electrode beneath it: in this way the
high-frequency signal is transmitted through the filter. 鈥淚t works like a
40-micrometre-long guitar string,鈥� Nguyen says. 鈥淧luck it and it reverberates at
specific frequencies.鈥� Frequencies that don鈥檛 correspond to resonances are
blocked.

Nguyen can adjust the frequency that the filter transmits by varying the size
of the beams鈥攕maller beams have a higher resonance frequency鈥攁nd the
position of the spring. And he can achieve even finer control by applying a bias
voltage to an electrode mounted beneath one of the anchors. This changes the
stiffness of the silicon structure, which in turn alters slightly the frequency
at which it resonates.

By combining lots of these filters in series or parallel, Nguyen can build
devices that block or transmit a wide range of different frequencies. So far,
his devices work at frequencies up to 70 megahertz. Now he is planning to expand
this frequency range into the more useful gigahertz regime, funded by a
$2.6 million grant that he has just received from the US Defense Advanced
Research Projects Agency (DARPA). 鈥淣guyen鈥檚 technology may be able to replace
all discrete radio components,鈥� says Albert Pisano, DARPA programme manager for
MEMS. 鈥淗e鈥檚 striking straight at the heart of the radio problem.鈥�

Besides their size, MEMS filters offer other advantages too. Since they are
mechanical devices, they use considerably less power than conventional filters,
and they should also be very cheap to mass-produce. These advantages have
already been recognised by other researchers, who have used MEMS technology to
build miniature inductors, voltage-tunable capacitors and micromechanical
switches.

Hot and high

But before we swap our pocket-size cellphones for MEMS transceivers, there
are still a number of significant problems to be overcome. Quartz filters
have one big advantage鈥攖hey are stable to changes in temperature, and any
drift in performance can easily be compensated for electronically. MEMS filters
are far more vulnerable to temperature change: as they warm up, their tiny beams
expand, changing the filter鈥檚 resonance frequency.

To deal with this problem, Nguyen is considering the use of miniature ovens.
These would consume a tiny amount of power鈥攏o more than a couple of
milliwatts鈥攖o keep the filter鈥檚 mechanical parts at a steady temperature.
Another problem, common with MEMS devices in general, is that they must be
encapsulated in a vacuum or under inert nitrogen gas to protect them from dust
and contaminants in the environment which could block their delicate
mechanisms.

However, Nguyen is confident that he can solve these problems: 鈥淚鈥檓
comfortable saying that past the year 2000 you might see these kinds of
transceivers on a watch,鈥� he says.

If these devices are going to change our lives, it may not be for a little
while yet. But Nguyen is looking forward to the challenges along the way. 鈥�
Star Trek explores the wider realms鈥攖he whole of space,鈥� he says.
鈥淏ut by trying to make things work at these sorts of tiny scales, we鈥檙e going in
the opposite direction. That鈥檚 the really sexy part of this research.鈥�