June 24, 1999 MURRAY HILL, N.J. -- The mainstay of the semiconductor revolution -- silicon-based transistor technology – may not run out of steam for roughly a dozen years, instead of the previous estimate of fewer than six years, report researchers at Lucent Technologies' Bell Labs in the June 24 issue of Nature.
In recent years, the semiconductor industry thought a limiting factor for producing increasingly smaller silicon-based transistors would be the crucial insulating layer. Made of silicon dioxide, the insulating layer on today's chips is typically 25 atoms thick, but the Bell Labs researchers have produced a five-atom-thick layer, the thinnest ever made. They also showed that a four-atom layer is the fundamental physical limit for silicon dioxide-based insulators.
The research results suggest that an alternative insulating material must be found before 2012. Or, if alternative insulating materials are not found, totally new semiconductor technologies will be needed.
"Having extended the fundamental physical limits of silicon dioxide gives the semiconductor industry more time to develop alternative insulating layers," said Bell Labs researcher David Muller, who used the most sophisticated electron microscopy technique available today to confirm the thickness results.
The insulating layer, known as the gate oxide, is the device's smallest feature. It lies between the transistor's gate electrode, which turns current flow on and off, and the channel through which this current flows. The gate oxide acts as an insulator by protecting the channel from the gate electrode, thus preventing a short circuit.
By continually reducing both the gate oxide thickness and the length of the gate electrode, the semiconductor industry has doubled the transistor's switching speed every 18 to 24 months, following what is known as Moore's Law.
"Bell Labs research on ultrathin gate oxides is delivering technology advances that we are applying to new generations of communications ICs," said Mark Pinto, chief technology officer for Lucent's Microelectronics Group. "These advances will enable us to provide system-on-a-chip capabilities with more performance and lower power consumption for such demanding applications as third-generation wireless systems."
To produce the ultrathin gate oxide, it was essential to "grow" atomic layers that were absolutely uniform and smooth. That's because the top and bottom layers were adjacent to the silicon itself, leaving only three layers in between. All three of the middle layers needed to be completely intact to prevent electrons from escaping through the gate oxide, which would lead to a short circuit.
Tackling the gate oxide challenge, researcher Greg Timp and his Bell Labs colleagues first cleaned the silicon and then used an unconventional process to add oxygen to the silicon to grow silicon dioxide. They used a process known as rapid thermal oxidation, which exposes the silicon to 1,000 degrees Celsius for 10 seconds.
To study the resulting gate oxide, which was later incorporated into a working transistor, Muller used a scanning transmission electron microscope. This analytical technique pinpoints individual atoms, examines how they bind to each other and also determines their electrical properties, which indicate insulating characteristics.
Because the Bell Labs findings are based on only research results, reliability and yield issues still must be explored before using these ultrathin gate oxides in a manufacturing setting.
Other Bell Labs researchers working on the project included Thomas Sorsch, Stephen Moccio, Frieder Baumann, and Kenneth Evans-Lutterodt. A portion of the microscopy research was done at Cornell University's Center for Materials Research.
Lucent Technologies, headquartered in Murray Hill, N.J., designs, builds and delivers a wide range of public and private networks, communications systems and software, data networking systems, business telephone systems and microelectronics components. Bell Laboratories is the research and development arm for the company. For more information on Lucent Technologies, visit the company's web site at http://www.lucent.com or the Bell Labs web site at http://www.bell-labs.com.
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