Dec. 9, 1998 SAN FRANCISCO -- Bell Labs researchers have developed a method to enhance the efficiency and reliability of system-on-a-chip products. They have replaced a uniform gate oxide thickness, which limits the chip's performance, with a gate oxide whose thickness varies among the chip's various components.
The research results were presented here today at the IEEE International Electron Devices Meeting.
In recent years, system-on-a-chip products have advanced integrated circuit flexibility and performance by putting different chip components, such as digital logic, analog circuitry and memory, on the same chip. This approach delivers dramatic increases in functionality, along with decreases in size and power consumption, and will enable such future products as wristwatch-sized single-chip cellular phones.
One potential limitation of the system-on-a-chip approach, however, has been the uniform thickness of the gate oxide for all chip components. For instance, memory, digital logic, and analog input/output (I/O) circuits ideally need gate oxides, which act as insulators within transistors, of different thicknesses for maximum efficiency and reliability because each component has different power requirements.
The only gate oxide is silicon dioxide, and it's literally grown as oxygen is added to a silicon surface. C.T. Liu and his Bell Labs colleagues were able to control the silicon dioxide's growth rate in different regions of the chip by injecting nitrogen into the silicon before the oxidation process. That's because the nitrogen retards the chemical reaction between silicon and oxygen by 20 to 80 percent.
"You can produce different thicknesses of the gate oxide," said Liu of Bell Labs, which is the research and development arm of Lucent Technologies, "by introducing varying amounts of nitrogen in different areas."
Liu and his colleagues successfully used two different approaches for the oxidation step. They not only used a conventional oxidation furnace at temperatures between 800 and 900 degrees Celsius for 20 minutes, but they also used rapid thermal oxidation, which exposed the silicon to temperatures above 1,000 degrees Celsius for about 10 seconds. The second approach, Liu said, improved both the quality and durability of the thin gate oxide because the higher temperatures create stronger chemical bonds.
Other researchers had attempted a similar nitrogen implantation process, but because they thought the nitrogen would damage the silicon, they attempted to anneal it immediately after nitrogen implantation. However, the nitrogen level in the silicon decreased more than 90 percent, and it was no longer possible to control the gate oxide growth. The process developed by the Bell Labs researchers does not require the annealing step.
An added benefit of achieving successful nitrogen implantation, Liu said, is reducing gate oxide degradation. That's because the chemical bond between nitrogen and silicon is stronger than a silicon-oxygen bond.
To test their new processing techniques, the Bell Labs researchers developed a high-performance test circuit with 0.18-micron features and multiple gate oxide thicknesses. With the same amount of power consumption, the circuit speed was twice as fast as the most recently published research results involving a uniform gate oxide thickness.
"Bell Labs advances in multiple gate oxide thicknesses are being included in our next-generation IC fabrication technology," said Mark Pinto, chief technology officer for Lucent's Microelectronics Group. "This kind of research makes it possible for us to deliver true system-level IC expertise -- with digital, analog, memory and now even RF capabilities -- to manufacturers of leading-edge network communications systems."
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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