gas-flow microvalve systems, and microbiological systems). There have also been several programs aimed at the commercial development of MEMS that have been discontinued, including those supporting automotive fuel-injection manifold air-pressure-sensing MEMS, because they were not found to be cost effective. In other applications, such as microvalving and suspension control, the adoption of MEMS has been slow. Displays based on MEMS, such as the mirror array by Texas Instruments (described below), also face intense competition from newly developed liquid-crystal designs. Although many observers regard these developments as normal growing pains for a new technology, others have serious reservations about the future of the field.

The remainder of this chapter presents an overview of current trends in the MEMS market. The chapter is divided into three sections. The first section describes MEMS that are already successful on the market, such as thermal ink-jet print-heads and accelerometers. The second section reviews MEMS technologies currently under development that show significant commercial potential, such as chemical-sensor arrays and display technologies based on mechanical reflecting elements. The third section discusses some future possibilities and long-range research opportunities.

COMMERCIAL SUCCESSES

Although most people still consider MEMS a technology of the future, a considerable number of people already use MEMS-based devices every day. The ink-jet cartridges in many commercial printers and many of the accelerometers used to deploy air bags in cars are MEMS devices. This section examines the commercial success of ink-jets and accelerometers.

Thermal Ink-Jet Printing

The thermal ink-jet print-head is the largest commercial success story for MEMS technology in terms of both unit sales and dollar amounts. Thermal ink-jet cartridges currently dominate the ink-jet printing market and account for well over a billion dollars per year, independent of the printers in which they are used. Ink-jet printers (both thermal and piezoelectric) typically cost less initially than dry-toner laser printers and, despite their slower speed and higher per-page cost, are often the solution of choice for low-volume print runs. Vendors of ink-jet printers include Canon, Epson, Hewlett-Packard (HP), Lexmark (formerly a part of IBM), and Xerox.

The concept of drop-on-demand thermal ink-jet printing was developed independently, and nearly simultaneously, by HP and Canon. HP commercialized the "Thinkjet" in 1984 using a glass substrate, while Canon commercialized its version as the "Bubblejet." Later print-heads used silicon

FIGURE 1-1 Cross-section of an integrated thermal ink-jet chip. This instantaneous view shows an ink droplet being ejected from the firing chamber by the "drive bubble" created by resistive heating. An NMOS (N-channel metal oxide semiconductor) transistor is associated with each firing chamber. Transistor addressing is achieved by a row-column address scheme. Source: Adapted from Beatty, 1996.

substrates to take advantage of the widely available equipment set and fabrication methods for silicon.

Thermal ink-jet print-heads (or pens) are packaged as replaceable drop-in cartridges on the order of 9 to 50 cm in volume. They usually comprise a supply of ink and an array of microscopic heating resistors on a silicon substrate mated to a matching array of ink-ejection orifices (Barth, 1995). In some designs, the associated active electronics are on the same substrate. These pens constitute the enabling technology-base for printers ranging from battery-powered, portable units to large-format bed plotters. Figure 1-1 shows a cross-section of a thermal ink-jet head with integrated active electronics. The orifice plate of the print-head is made of plated nickel laminated on top of a polymer barrier layer. Although producing this arrangement requires a departure from purely lithographic batch processing, the lamination process has been demonstrated to be cost effective for the large volumes demanded by the ink-jet market.

Figure 1-2 illustrates the decrease in ink-drop weight over time for one family of ink-jet printers. Image quality is greatly

FIGURE 1-2 Evolution of ink-jet drop weight versus time. Drop weights below the dotted line can produce photographic-quality images. Source: Beatty, 1996.



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