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Lewis Acid Stabilizes Surface of Light-Emitting Porous Silicon Jillian Buriak, assistant professor in chemistry at Purdue University, has developed a way to stabilize the surface of porous silicon, a light-emitting material. According to an article published in the February 17 issue of the Journal of the American Chemical Society, Buriak coats

the porous surface of the silicon with a Lewis acid, a solution that brings about a reaction that produces a hydrophilic coating that protects the surface while allowing the porous silicon to maintain its photoluminescent properties. To test how well the treatment stands up to environmental stresses, Buriak boiled samples of treated and untreated porous silicon in a highly basic solution of potassium hydroxide for an hour. "Silicon and silica compounds generally dissolve in a solution with a pH greater than 7," she said. "By boiling it, we are accelerating the aging process to test how well this stabilizing method will stand up to rigorous conditions over a period of time." The treated surfaces showed no oxidation and only minor changes in photoluminescent properties, while the surfaces of the untreated samples dissolved. "This indicates that, once it is treated, the surface will remain stable for long periods of time," she said. The new treatment will also allow scientists to add other compounds to the surface, so that the light-emitting properties of porous silicon can be manipulated to respond to certain chemicals or conditions. Buriak said, "When uv light strikes the surface of porous silicon, it reradiates back in the red wavelength, producing a bright orange color. But if we add, for example, a chemical that binds to sodium ions, when sodium is present it will cause the reradiated wavelength to shift, producing a different color such as yellow or red. So you could look at the color difference and see whether sodium is present, and at what concentration if s present."

Ultrathin Film Si-Based Circuits Fabricated on Polyimide Substrate A process demonstrated by researchers from Iowa State University's Microelectronics Research Center and Mankato State University in which crystalline and noncrystalline thin-film silicon-based circuits are deposited on polyimide has created working integrated circuits less than 5-^m thick. Currently, devices of similar dimension (lengths of 2-6 }im) are on substrates 400 Jim in depth.

According to a paper published in Electrochemical Society Proceedings Volume

94-35, the research team began the process by using a spin-on technique to coat a rigid 100-mm oxidized n-type silicon wafer with 6-^m-thick polyimide. The film is then cured in several stages at temperatures up to 350°C. This approach produces the surface needed for photolithography while easing the removal of the film and circuit. After application of polyimide to the wafer, the surface of the polyimide is treated to promote adhesion of the electronics. A thin oxide layer is then deposited onto the treated polyimide. This layer acts to electrically isolate the electronics from th