Silicon's days are far from numbered
04/01/2003
By Robert P. Donovan
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Back in December, CleanRooms Chief Editor Mike Levans e-mailed me a report on stable organic polymers research presented at the 2002 Materials Research Conference (MRS) in Boston last fall.
The lead-in for this report was something to the effect that stable organic polymer films exhibiting semiconductor properties could now be made by simple deposition techniques such as ink printing, casting or spin-coating, and perhaps without the need for a cleanroom environment during manufacturing.
Since organic films have already been successfully fabricated into a variety of semiconductor devices, including light-emitting diodes and thin-film field effect transistors, and since integrated structures containing both optical and electronic components on a common substrate have already been produced,1 did this announcement herald the beginning of the end for the silicon dynasty in electronics and the cleanroom market it represents?
After all, these printed organic films can be deposited on many different types of substrates, including flexible substrates, at near room temperature in an unprotected laboratory environment,2 meaning that their capital requirements and production costs are, at least potentially, far, far lower than those of today's silicon technology.
What's the likely impact of these recent, startling organic material developments on the future of cleanrooms?
Over the next few years, probably not much; and thereafter, perhaps the developments may even be a boost to cleanroom manufacturers.
While a few commercial electro-luminescent displays already employ organic semiconductors—this application area is growing rapidly—semiconductor memories and microprocessors remain exclusively the province of silicon technology.
In these applications, organic semiconductors are unlikely to become cost-effective for a long time, if ever.
In a recent presentation at Sandia National Laboratories, Alan Heeger, one of the three winners of the 2000 Nobel Prize in chemistry for the discovery and development of conductive polymers, advised, "Don't sell your Intel stock." At least, not yet. This advice comes in spite of the very impressive organic device performance he reported, and actually demonstrated, as well as the glowing future he predicted for such devices.
Clearly, the promise of organic semiconductors in many applications is undeniably encouraging, but as more of a complement to silicon technology than a head-on challenger.
Another consideration to keep in mind: Conduction mechanisms in organic films are not yet fully understood but have been shown to be sensitive to trace impurities. In turn, impurity control is likely to be just as important in manufacturing organic semiconductor devices as it is for inorganic semiconductor devices.
Thus, control of the manufacturing environment of organic semiconductors will probably remain an important parameter as will control of the impurity concentration in the materials used to form the organic films.
The room temperature carrier mobility of even the most developed organic semiconductors still lags by several orders of magnitude behind that that can be routinely achieved in single crystal silicon and gallium arsenide devices.
In addition, silicon technology itself does not stand still. Equally startling reports of light emission from silicon devices—once thought to be impossible—are becoming more numerous.
About the same time that the MRS conference heard about the promising advance in the stability of organic polymer semiconductor films, a group in Italy announced improved light emission efficiency from a silicon-based structure.3
According to a report from the group in Italy, "The quantum efficiencies achieved are about 100 times better than has previously been possible with silicon and are, for the first time, comparable to those obtained from GaAs."
Light emission from silicon structures holds out the promise of being able to combine optical and electrical functions on a single silicon chip, a breakthrough of perhaps even more significance than that of incorporating both optical and active electrical organic components on a single substrate.
Silicon's days are far from being numbered. And silicon manufacturing alone implies continuation of the need for clean manufacturing environments, whether ballroom cleanrooms, minienvironments or cluster tools are used to achieve them. So make sure to retain your investments in cleanrooms and cleaning technology along with your Intel stocks. lll
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories and a monthly columnist for CleanRooms magazine. He can be reached at [email protected].
References
1. Sheraw, C. D. et al, "Organic Thin-Film Transistor-Driven Polymer-Dispersed Liquid Crystal Displays on Flexible Polymeric Substrates," Applied Physics Letters 80(6), 11 February 2002, pp. 1088-1090.
2. 2nd Annual Flexible Microelectronics & Displays Conference, Phoenix, Ariz., February 3-4, 2003 [[email protected]].
3. "World Record for Silicon Light-Emission," Physics Web, 5 November 2002 [http://physicsweb.org/article/news/6/11/2].