Strained Silicon Could Extend Limits of CMOS Technology

Semiconductor industry leaders are still warning that Moore’s Law–the doubling of the amount of transistors on a computer chip every 18 months–is leading the semiconductor industry into an impending crisis as computer chips approach fundamental physical limits. Specifically, higher densities, faster speeds and smaller sizes mean that computer chips will soon be generating more heat as they operate than scientists can remove. But research that physicists started some 20 years ago is coming to the rescue, according to UCLA’s Ya-Hong Xie, a featured speaker at the APS March meeting.

Xie is a pioneer in the area of strained silicon–essentially a stretched-out form of silicon–which he believes is an excellent way to make faster, low-power computer chips with conventional CMOS technology. In fact, strained silicon technology is one of the hottest technologies in the IC industry, already appearing in the product lines of major chip manufacturers such as Intel, AMD, Texas Instruments and IBM.

Strained silicon essentially trades off speed with power to address the heat problem. By stretching a thin layer of silicon, two of its six electrons drop to a lower energy level, so it can achieve the same amount of conductance at lower power. However, to be effective, the sample material must be perfectly homogenous. Xie uses epitaxy to grow his materials layer by layer. He starts with a layer of silicon germanium (SiGe) layer. The top layer "relaxes" and a silicon layer is grown on top of that. This second layer has a large lattice constant–that is, it is "strained"–because the new layer tends to take on the structure of the layer immediately beneath it.

Also at the meeting, George Celler, chief scientist at the semiconductor manufacturer SOITEC described silicon-on-insulator (SOI) technology as another solution for making faster chips. SOI has raw speed, up to 30% faster than bulk silicon, a gain of an entire chip generation. It also consumes less power and has lower heat so the chips don’t melt. And it can incorporate strained silicon technology.

Thus, SOI may be the key to faster, cooler chips, reducing heat for the same amount of power. Celler predicts a billion dollar SOI market by 2008. There are many fabrication facilities for SOI currently under construction around the world, and the next generation of game machines–Sony’s Playstation 3 and Xbox Next from Microsoft–will use SOI substrates.

Ralph Cavin, president of SemaTech, said that the IC industry expects to reach the technological limits of silicon by 2015-2018, when the dimensions of transistor gates are only seven times smaller than they are today. There is a great deal of research and development focused on new technologies: spintronics, molecular electronics, and further out, quantum computing. But he also insisted that silicon isn’t going anywhere: CMOS is a $200 billion per year market.