Dec. 8, 1998 SAN FRANCISCO -- Bell Labs researchers have created and proven a theory that explains why gate oxides -- a critical component in transistors and flash memory applications -- wear down and eventually become unreliable.
The research findings, which include novel simulation software, could help Lucent's Microelectronics Group and the rest of the semiconductor chip industry find ways to maintain the reliability of electronic devices, even as gate oxide layers continue to be made thinner and thinner as chips shrink in size. The researchers presented their findings Dec. 7 at the IEEE International Electron Devices Meeting.
In a transistor, the thin gate oxide layer -- usually silicon dioxide -- lies between the gate electrode, which turns current flow on and off, and the channel through which this current flows. The gate oxide layer, in essence, acts as an insulator, protecting the channel from the gate electrode and preventing a short circuit.
By reducing the thickness of the gate oxide layer, it is possible to increase the transistor's switching speed. That's because the electrode is even closer to the channel, thereby inducing a larger current to flow through the transistor. However, the thinner oxide layers degrade at lower voltages, and their wear-out behavior is more difficult to understand than the behavior of thicker oxides. As a result, it is quite difficult to predict both the lifetime of a thin gate oxide and which voltages are safe for transistor operation.
To better understand these issues, Bell Labs researchers Jeff Bude, Bonnie Weir and Paul Silverman studied gate oxide thicknesses -- less than 20 atoms thick -- in today's 0.25-micron transistors. Previous theories of oxide damage suggested a weak flow of electrons through the oxide layer could knock lower energy electrons out of their places, leaving behind "hot" holes -- regions without electrons -- which then could penetrate the gate oxide and weaken it. However, this theory was insufficient to evaluate thinner oxides and also oxide wear-out at lower voltages.
The three researchers corrected the deficiencies in the standard theory and developed software that simulates the hot hole phenomenon and predicts the exact location and energy of the holes. They then compared the findings with observations of gate oxides that had been damaged, and the two sets of results matched.
"During the design process for gate oxides," Bude said, "the software could predict when the hot-hole phenomenon could pose a problem, and the designers could try a different approach. The software also could predict how much voltage a gate oxide can withstand."
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.
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