For Release: April 12, 2001
Media Relations Office
Researchers at the NASA Glenn Research Center, Cleveland, OH, have received a second patent for their method of growing atomically flat surfaces, without a single step even one atom high, on commercial semiconductor wafers. The step-free surfaces hold particular promise for improving the performance and reliability of a new class of microelectronic devices that could be used in everything from DVDs to jet engines.
Commercial semiconductor wafers are covered with steps, typically one to eight atoms high, despite careful polishing by their manufacturers. The Glenn research team makes step-free surfaces by first etching device-sized arrays of mesas into the wafers. Next, by controlling conditions, the researchers limit crystal growth to the riser, or side, of each atomic step. The crystal at each step grows sideways until the step reaches the edge of the mesa, leaving behind an atomically flat surface.
"We've flattened silicon carbide mesas as large as 0.4 by 0.4 millimeter and, depending on the mesa size, over half of the mesas on a wafer," said Tony Powell, senior research physicist at Glenn. "What's so attractive about our method is that, with just one extra patterning step in the fabrication, manufacturers can make these step-free surfaces," he said.
Those mesas that were not made flat contained screw dislocation defects -- so called because of the warped spiral stacking of the crystalline planes -- which are not amenable to this flattening method. An added benefit of the method is that it isolates the screw defects into mesas that can be identified and avoided.
Studies by other scientists have linked surface steps in the wafer to defects in semiconductor films that are different from the wafer material. The defects cause poor performance and reliability and have been troublesome to the development of new electronics for aerospace applications.
"We believe that step-free surfaces will enable remarkable improvements in devices based on silicon carbide and gallium nitride . These are the materials of choice for making high power solid-state switches as well as electronics for hostile environments, such as pollution and noise control devices inside aircraft engines," Powell said.
Industry researchers are using these materials for blue light emitting diodes (LEDs) for lighting, blue lasers for higher capacity DVDs and high efficiency transistors for more reliable electrical power switching and improved wireless communications.
Glenn is one of the Nation's leading aeropropulsion research laboratories and conducts instrumentation and controls research on sensors, electronics, photonics and microelectromechanical systems (MEMS) for aeronautics and space applications.
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