Could carbon be silicon's best friend?

Cutting silicon into individual wafers is a precise and expensive process that creates about 25 percent waste, which drives up costs to semiconductor manufacturers. A materials science engineer at Washington University in St. Louis plans to test a new cutting material made of diamond-coated carbon fibers to make the process more efficient and cost-effective.

Figure (a) shows a spool of diamond-coated wire interlaced through the cutting grooves of a sawing machine. Figure (b) is a cross section of the cutting groove and shows the cutting action.

Parag Banerjee, assistant professor of materials science in the School of Engineering & Applied Science, has received a three-year, $300,000 Grant Opportunity for Academic Liaison with Industry (GOALI) from the National Science Foundation to work with SunEdison Inc., a Maryland Heights, Mo.-based renewable energy development company, to find a more efficient way to cut silicon into wafers using carbon fiber and diamonds.

SunEdison, one of the largest manufacturers of silicon wafers worldwide, uses the wafers in solar panels. Silicon wafers are cut from silicon ingots, which are about 15 centimeters by 15 centimeters square. Cutting the ingots into wafers comprises about 11 percent of the final cost of solar energy panels. Reducing waste and cost to cut the wafers could ultimately reduce the cost of solar energy panels.

The sawing machine used to slice the silicon ingots into wafers consists of a row of stainless steel wires, each about 100 microns in diameter –about the width of a cat whisker. The silicon ingot is pressed on the wires as these move back and forth at high velocity. However, this process creates waste, which may be reduced by making steel wires with thinner diameters. Yet making steel wires thinner than 100 microns is not a viable option, since wires become weaker when thinner and cannot tolerate the force needed to cut through silicon.

With the funding, Banerjee and his lab will test cutting silicon ingots with carbon-fiber wires that are 75 microns in diameter to see if the thinner, carbon fiber wires can stand up to the force needed to slice the wafers.

What makes the wires able to cut through the silicon is a coating of diamonds – not the kind that sparkle in jewelry, but a lab-created diamond powder that coats the wires to improve cutting strength. The wires also are coated with another form of carbon.

"Part of the project will be looking at different recipes on how to make the diamond stick to the carbon fiber wires and looking at the mechanical strength of the wires with the diamond coating," Banerjee says.

"There are fundamental materials issues that our lab will seek to answer, but ultimately, the real test of this effort would be for SunEdison to develop the process at an industrial scale," Banerjee says. "This is where the NSF's funding mechanism through GOALI plays a crucial role: By creating a pipeline for laboratory-based work to be harnessed by industry, the GOALI program ensures engineering research is immediately relevant and impactful to society."

The grant allows one graduate student from Banerjee's lab to work on the project and closely interact with SunEdison process engineers. In addition, Banerjee plans to hire an undergraduate student to work on the project each of the next three summers.

The School of Engineering & Applied Science at Washington University in St. Louis focuses intellectual efforts through a new convergence paradigm and builds on strengths, particularly as applied to medicine and health, energy and environment, entrepreneurship and security. With 88 tenured/tenure-track and 40 additional full-time faculty, 1,300 undergraduate students, more than 900 graduate students and more than 23,000 alumni, we are working to leverage our partnerships with academic and industry partners — across disciplines and across the world — to contribute to solving the greatest global challenges of the 21st century.