Plasma-assisted polishing smooths single-crystal diamond

Japanese researchers have found a way to polish the hardest known material without damaging it, helping to accelerate its use in advanced electronics.

Silicon has been the workhorse of electronics for decades because it is a common element, is easy to process and has useful electronic properties. However, one issue with silicon is that high temperatures damage it, which limits the operating speed of silicon-based electronics. And while single-crystal diamond is a possible alternative to silicon, common methods of polishing the surface — a requirement for its use in electronics — are a combination of slow and damaging.

Now researchers from Japan’s Osaka University, the National Institute of Advanced Industrial Science and Technology and TDC Corporation have managed to polish a single-crystal diamond wafer to be nearly atomically smooth. Published in the journal Scientific Reports, the procedure will be useful for helping diamond replace at least some of the silicon components of electronic devices.

Diamond is the hardest known substance and essentially does not react with chemicals. Polishing it with a similarly hard tool damages the surface and conventional polishing chemistry is slow. In the new study, the researchers in essence first modified the quartz glass surface and then polished diamond with modified quartz glass tools.

“Plasma-assisted polishing is an ideal technique for single-crystal diamond,” said lead author Nian Liu, from Osaka University. “The plasma activates the carbon atoms on the diamond surface without destroying the crystal structure, which lets a quartz glass plate gently smooth away surface irregularities.”

The single-crystal diamond, before polishing, had many step-like features and was wavy overall, with an average root mean square roughness of 0.66 µm. After polishing, the topographical defects were gone and the surface roughness was a mere 0.4 nm.

Shape of mosaic single-crystal diamond substrate before and after plasma-assisted polishing. Image credit: Osaka University.

“Polishing decreased the surface roughness to near-atomic smoothness,” said senior author Kazuya Yamamura, also from Osaka University. “There were no scratches on the surface, as seen in scaife mechanical smoothing approaches.”

Furthermore, the researchers confirmed that the polished surface was unaltered chemically. For example, they detected no graphite, which means no damaged carbon. The only detected impurity was a very small amount of nitrogen from the original wafer preparation.

“Using Raman spectroscopy, the full width at half maximum of the diamond lines in the wafer were the same, and the peak positions were almost identical,” Liu said. “Other polishing techniques show clear deviations from pure diamond.”

The breakthrough means that high-performance power devices and heat sinks based on single-crystal diamond are now attainable, the researchers claim. Such technologies should dramatically lower the power use and carbon input, and improve the performance, of future electronic devices.

Top image credit: ©stock.adobe.com/au/htoto911

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