Page 137 - An Introduction to Microelectromechanical Systems Engineering

116 MEM Structures and Systems in Industrial and Automotive Applications

the silicon nitride layer to further increase thermal isolation and improve the sen-
sor’s performance.
The operation of the earlier sensor by Motorola consists of applying to the
heater a 5-V pulse for 5s, followed by a 1-V pulse lasting 10s. The corresponding
temperature is 400ºC during the first interval, decreasing to 80ºC during the second
pulse. To maintain consistency, the resistance measurement always occurs at the
same time during the interval—in this case, at 9.5s into the second 10-s long pulse.
The MiCS sensor demonstrates a response from 10 to 1,000 parts per million (ppm)
of carbon monoxide (CO) over a humidity range of 5 to 95%. The output signal
shows a square-root dependence on CO concentration, with little dependence on
humidity for CO concentrations above 60 ppm.

Actuators and Actuated Microsystems

The physical world is not still but is rather very dynamic and full of motion. If sen-
sors extend our faculties of sight, hearing, smell, and touch, then actuators must be
the extension to our hands and fingers. They give us the agility and dexterity to
manipulate physical parameters well beyond our reach. It is not surprising that the
promise to control at a miniature scale is fascinating. Wouldn’t the surgeon dream of
electronically controlled precision surgical tools? And what to do when our sensors
tell us of a need to locally act and control on a microscopic scale? It is actuation that
affords us the ability to apply this type of feedback.
In this section, we address the use of micromachined actuators primarily in
industrial and automotive applications—though it is well understood that with
minor modifications, these actuators can be applied to other markets. Inkjet heads
and microvalves are perhaps the most notable examples to discuss. Micromachined
pumps are also emerging as new products of the future. The complexity of actuated
microsystems continues to increase as the technology matures, accompanied with a
rapidly rising level of integration. For instance, novel microfluidic systems now inte-
grate valves and pumps, as well as various types of sensors and interconnecting
channels, and they have become a separate field of study and development [35].

Thermal Inkjet Heads
The thermal inkjet print head, ubiquitous in today’s printers for personal comput-
ers, receives frequent mention as a premier success story of MEMS technology.
While thermal inkjet technology is a commercial success for Hewlett-Packard, Inc.,
of Palo Alto, California, and a few other companies, there is little in it that origi-
nates from silicon MEMS per se. Early generations of inkjet heads used electro-
formed nickel nozzles [10, 36, 37]. More recent models use nozzle plates drilled by
laser ablation [38]. Silicon micromachining is not likely to compete with these tra-
ditional technologies on a cost basis. However, applications that require high-
resolution printing will probably benefit from micromachined nozzles. At a resolu-
tion of 1,200 dots per inch (dpi), the spacing between adjacent nozzles in a linear
array is a mere 21 µm. A greater number of laser-drilled nozzles on a head raises the
cost, while the cost remains constant as holes are added using batch-fabrication
methods. Nonetheless, the nozzles continue to be made in nickel plates, but
micromachining technology is now necessary to integrate a large number of