Electro-Optics Made Easier

David L. Shenkenberg

Doctoral candidate Çaglar Ö. Girit and professor Alex Zettl from the University of California, Berkeley, have developed an alternative to electron-beam lithography, which is used for making the nanoscale electrical contacts in the silicon microprocessors found in numerous electronic devices, including computers, cell phones and audio players. They call the technique “nanosoldering.”

Nanosoldering can be used not only for establishing nanoscale electrical junctions in silicon microprocessors but also in photodiodes and semiconductor lasers that are extremely small, saving space for optical designers. The technique also works with next-generation materials such as graphene, a form of carbon. A hypothetical single layer of graphene in air on a silicon-oxide substrate can conduct more than 1000 times the current of a superconductor. All that power can make for one very fast computer, or a semiconductor laser with greater output power and fewer losses.

Researchers developed a nanoscale soldering technique that can make nanoscale electrical junctions in processors and photonic components made of silicon and of next-generation materials. The technique is based on creating this spike of solder less than 100 nm in size.

Although lithography can make nanoscale electrical contacts in silicon and other materials with high quality and repeatability, the technique is complex, time-consuming and expensive, the researchers said. “The cost to install and maintain an electron-beam lithography system runs well into the millions of dollars,” Girit said. “Implementation of our technique costs at most one percent of that sum.” Furthermore, lithography generates residues that can contaminate and, thus, reduce the efficacy of electrical components, whereas nanosoldering does not.

To perform nanosoldering, the investigators placed the solder and the graphene in separate areas on a flat heated surface on a stage under an optical microscope from Olympus America Inc. of Center Valley, Pa. They used the heated surface to melt the metals they employed as the solder, including indium and low-melting-point alloys of indium and tin. Graphene has a higher melting point, so it was not affected. To pick up the bead of melted solder, they used a micromanipulator from Newport Corp. of Irvine, Calif. In so doing, the solder tapered to a spiked tip smaller than 100 nm. Girit compared this process to the way a glassblower pulls out very thin filaments from molten glass. The researchers next held the spiked solder tip above the graphene and quickly raised the stage, so that the solder connected the graphene. They removed the manipulator and turned off the heat on the stage, and the solder solidified.

The soldered graphene conducted current at densities as high as 500 A/m without fail. The scientists said that this current density is two orders of magnitude greater than that of existing silicon microprocessors, assuming uniform power dissipation.

Girit said that an existing technique, called wirebonding, can be used in conjunction with nanosoldering, skipping the lithography steps, for a totally automated procedure requiring no operator.

The researchers believe that they could improve their technique by doing it inside a scanning electron microscope, because they could see the samples more easily as a result of the higher spatial resolution of the instrument. The accuracy of the manipulator also could be improved, Girit said.

Applied Physics Letters, Nov. 5, 2007, 193512.