Superwicking with silicon

Margaret W. Bushee, margaret.bushee@photonics.com

Because most computer chips and transistors are made of the semiconductor silicon, the element is key to the microelectronics industry. A serious barrier with silicon, however, is keeping components cool. Until now, computer chips have been air-cooled, but air’s limited capacity to absorb heat has been a hurdle in the quest to design ever-faster computer chips.

That obstacle may soon be overcome, thanks to scientists at the University of Rochester’s Institute of Optics, whose research on high-intensity laser-treated silicon has strongly suggested a new way to cool components: using water or volatile liquids. By patterning parallel grooves onto small silicon wafers and testing how the liquids flow over them, they discovered that these liquids are propelled upward in a gravity-defying way described as “supercapillary action” and “superwicking.”

(A) On this silicon wafer, an array of laser-carved parallel grooves appears black. This type of laser treatment would enhance heat absorption and could benefit the solar industry. (B) Shown is a scanning electron microscopy image of microgrooves.

The laser treatment renders the silicon superhydrophilic; i.e., the liquid molecules become more attracted to the silicon than to each other. In stark contrast, liquids bead up on the surface of silicon in its normal state. Associate professor of optics Chunlei Guo and his assistant Anatoliy Y. Vorobyev published their study in the March 15, 2010, issue of Optics Express.

Groovy travels

The researchers used an amplified Ti:sapphire laser to generate 65-fs pulses at a maximum repetition rate of 1 kHz. The beam was focused onto a 25 x 25 x 0.65-mm silicon wafer, creating a 22 x 11-mm array of parallel microgrooves spaced 100 µm apart. Each groove was 22 mm long and approximately 40 µm deep. Magnified microscopy views show that within the microgrooves are nano- and microstructures, and that the nanostructures consist of both nanoprotrusions and nanocavities.

In the pivotal experiment, they placed a wafer on a table at a perpendicular angle and pipetted a droplet of water onto the base of a microgroove. At a top speed of 3.5 cm/s, the water traveled uphill over the groove’s 22-mm length, slowing down slightly over time.

Motivating the study was research performed by Guo and Vorobyev a year earlier, using a similar laser patterning technique but with metals – platinum and gold. The outcome at that time was that the patterned surface pumped liquids uphill. Testing the metals for wettability, the investigators found that laser treatment renders the surfaces hydrophilic. The article, titled “Metal pumps liquid uphill,” was published in the June 1, 2009, issue of Applied Physics Letters.

The results of the 2009 study were a bit of a surprise but a fortunate outcome in terms of silicon development. “Our 2009 study in turning metals superphilic was a bit unexpected, but we were determined to work on silicon afterwards,” Guo said.

He anticipates that the superwicking technique may find numerous applications, including nano-/microfluidics, lab-on-a-chip technology, chemical and biological sensors, and solar cells.

As for the future, Guo said, “We plan to further study the fundamental mechanisms behind our observation. We also would like to look into applying our technique in real-life applications.”