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October 7, 2009 - Researchers at French R&D institute Leti say they have overcome incompatibilities of metallic catalysts with CMOS manufacturing in order to make silicon nanowires, bridging a gap between CMOS technology and bottom-up growth of nanowires and enabling new functionality to be added to chipmaking processes.
Semiconductor nanowires' high surface-to-volume ratio makes them excellent candidates for sensors (electrical detection of chemical/biological substances) and solar energy production, and their small mass is valuable for mechanical mass detection. They also have potential for adding functions such as sensing and energy production on top of ICs. A roadblock to their use in IC production, though, has been the issue of CMOS compatibiltiy -- many metallic catalysts used for nanowire growth aren't compatible with standard CMOS fabrication processes, and those metals that are compatible (e.g., aluminum and copper) require temperatures above the maximum allowed temperature (450ºC). Researchers, assuming that oxidized metals are unsuitable for nanowire synthesis, have tried to remove the oxides.
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| Figure 1. Silicon nanowires obtained by the new method. (Source: Leti) |
In a paper published by Nature Nanotechnology, Leti says it turned to the "unconventional preparation method" of oxidizing a copper catalyst precursor, and using the reaction's high chemical activity to help reduce the nanowire synthesis temperature to as low as 400ºC -- allowing the growth of silicon nanowires with CMOS-compatible catalyst and temperatures. The work can be performed on standard deposition tools, noted Vincent Jousseaume, co-author of the paper, in an e-mail exchange with Small Times.
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| Figure 2. Image taken of a tomographic 3D reconstruction illustrating the morphology of the tip of a nanowire (~70nm dia.) after oxidation. Silicon-containing regions appear in blue; copper-rich regions appear in red. (Source: Leti) |
What's the forseen application of synthesized nanowires in IC manufacturing? "There will be first the possibility of introducing logic in the interconnect layers," in principle allowing fabrication of vertical transistors in vias, a step toward reconfigurable interconnect, according Jousseaume. There is also the promise of adding functionality beyond ICs, sucha s chemical or bio sensors to mechanical nano-actuators and embedded solar cells to provide power directly to the circuits, he said.
The next step in the work is to demonstrate applications for the technology, and optimizing the nanowire growth -- e.g., better controlling the nanowire orientation with respect to the substrate, Jousseaume said.
