Hybrid Chips of Gallium Nitride and Silicon
Researchers at MIT say they’ve made a big step toward combining the capabilities of the silicon used in computer chips with properties of the compound semiconductors found in lasers and high-powered electronics. In the October issue of IEEE Electron Device Letters, they report having figured out how to add devices made from compound semiconductors to a silicon chip, in a way that’s compatible with standard silicon manufacturing processes.
Compound semiconductors, made of two or more elements—gallium nitride (GaN) or indium phosphide, for example—have attractive properties that silicon lacks. They can make faster transistors than silicon, handle more power, and emit or collect light more easily. For its part, silicon is inexpensive and plentiful, and decades of development have led to processes for making complex, nanoscopic circuits in silicon. It would be useful to mix devices made out of different materials on the same chip, but differing physical characteristics make it difficult to grow compound semiconductors on top of silicon.
Rather than try to grow another semiconductor on silicon, Tomás Palacios, an assistant professor of electrical engineering and computer science at MIT, opted to make a separate wafer of GaN and bond it to the silicon. He then uses standard photolithography to create circuits in both the silicon and the GaN. “It basically for the first time allows circuit and system designers to choose the best semiconductor material for each device they want in the chip,” Palacios says. “They don’t have to compromise anymore.”
The basic problem with growing any nitride on the kind of silicon used in ICs is that the crystal lattices in each material are oriented differently, which causes defects to spread through the nitride that would render it useless. Palacios and his team grew a wafer of aluminum gallium nitride/gallium nitride on a different type of silicon, one cut along a crystal lattice that renders it useless for ICs but good for growing GaN. They coated the top of the nitride with a thin layer of an oxide called hydrogen silsesquioxane, 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, gluing them together. Next, they etched away the unusable silicon at the bottom, then used standard processes to inscribe circuits on the good silicon on top, being careful to leave 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.
Palacios says the process should work with any nitride combination, because nitrides, unlike other compound semiconductors, can withstand the 1000 °C temperature used in processing silicon. Nitrides could also be used to build LEDs or lasers on chips for use as high-speed optical interconnects, he says. Manufacturers could cut cost and size by combining several chips into one and also increase processing speed while cutting down on energy use and waste heat.
One challenge that remains is increasing the size of the nitride wafer. Right now they can be made only 25 millimeters (1 inch) in diameter, so only a few chips could be made on each wafer. They’d have to reach at least 152 mm (6 inches) in diameter to make the process commercially viable. Palacios expects that the technology could be commercialized within about two years.
Jeremy Muldavin, assistant leader of the Advanced Silicon Technology Group at MIT’s Lincoln Laboratory, who was not involved with Palacios’s work, says the process seems well suited for applications that mix systems on a single chip, as long as they require only a few nitride devices. Having two layers of material, and having to reduce the area of silicon to expose the gallium nitride could impose some design limits on the chips, he points out, but it could also have significant electrical advantages. “This work is an important step along the ‘beyond silicon’ path,”Muldavin says.
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