There have been several recent proposals from research institutes around the world on techniques to circumvent the challenges that make it harder to keep up with the Moore’s Law schedule.
There is a growing realisation that the industry is approaching the physical limits of what is achievable with further miniaturisation.
Researchers at the Massachusetts Institute of Technology (MIT), in a paper titled ‘Seamless On-Wafer Integration of Si MOSFETs and GaN HEMTs,’ published in the October 2009 issue of IEEE Electron Device Letters, have proposed a promising new solution that involves combining semiconductor materials that possess different and potentially complementary properties to form a new generation of hybrid microchips.
Compound semiconductors such as gallium nitride (GaN) or indium phosphide have properties such as being faster or being capable of handling higher power and higher frequencies that make them superior to silicon for a variety of applications. However, silicon offers complex integration, scalability and lower costs that have helped it replace most complex semiconductors in a large number of markets that place a premium on integration and costs.
According to the researchers though, modern chips do not require every transistor to be high-speed and high-performance. Only about 5% to 10% need to be able to perform at peak. In the paper, the assistant professor of electrical engineering at MIT, Tomas Palacios and his graduate student, Will Chung, explain how they have integrated GaN into a wafer with silicon transistors.
The crystal lattices of the two materials are oriented differently, and this would normally cause defects to spread through the nitride. The team grew a wafer of aluminium gallium nitride/GaN on a type of silicon cut along a crystal lattice that renders it useless for integrated circuits (ICs) but good for growing GaN, and coated the top of the nitride with a thin layer of hydrogen silsesquioxane. They then pressed a wafer of electronics-grade silicon wafer down on top of that and heated the wafer sandwich to 400°C for an hour, effectively integrating the two wafers. They then etched away the unusable silicon at the bottom and used photolithography to etch circuits on the silicon, while leaving some of the wafer blank. After that, they removed silicon from the blank areas of the wafer, exposing the nitride, and inscribed circuits in that.
The result was a fast-switching GaN device, called a high electron mobility transistor, right next to an ordinary field effect transistor made of silicon. The nitride wafer can currently only be made up to 25 millimetres in diameter, thereby limiting the number of chips that can be made on a single wafer. This needs to be scaled up by at least six times before the technology can be commercialised.
Recently, a team of researchers at the University of Florida led by Timothy Anderson, distinguished professor at the chemical engineering department of UFL, and Olga Kryliouk, research associate professor at the same department, had announced a novel method to grow GaN on silicon substrates. The achievement at MIT is distinct in that it instead actually embeds the GaN device into a silicon device.
Such hybrid chips can be manufactured using the standard technology that is already used today, and promises to herald a new era of integrated technology that could include hybrid chips that combine lasers and electronic components or devices that harvest energy from vibrations in the environment and silicon components that run on this energy.
For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)21 680 3274, [email protected], www.frost.com
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