London, May 5 : Princeton engineers have overcome the tiny defects and performance barriers associated with microchip shrinking, by developing a new technique that could considerably improve chip quality without increasing fabrication cost.
The team says that the method enables more precise shaping of microchip components than what is possible with current technology.
These component shapes could help manufacturers build smaller and better microchips, the key to more powerful computers and other devices.
"We are able to achieve a precision and improvement far beyond what was previously thought achievable," Nature quoted electrical engineer Stephen Chou, the Joseph C. Elgin Professor of Engineering, who developed the method along with graduate student Qiangfei Xia, as saying.
Microchips work best when the structures fabricated on them are straight, thin and tall. Rough edges and other defects can degrade or even ruin chip performance in most applications.
"These chip defects pose serious roadblocks to future advances in many industries," Chou said.
To deal with this problem, researchers try to improve the process used to make the microchips. However, Chou said such an approach works only to a point; eventually chip makers will run up against fundamental physical limits of current manufacturing techniques. "What we propose instead is a paradigm shift: Rather than struggle to improve fabrication methods, we could simply fix the defects after fabrication. And fixing the defects could be automatic -- a process of self-perfection," said Chou.
Chou's method, termed Self-Perfection by Liquefaction (SPEL), achieves this by melting the structures on a chip momentarily, and guiding the resulting flow of liquid so that it re-solidifies into the desired shapes.
This is possible because natural forces acting on the molten structures, such as surface tension, the force that allows some insects to walk on water, smooth the structures into geometrically more accurate shapes.
Simple melting by direct heating has previously been shown to smooth out the defects in plastic structures. This process can't be applied to a microchip, for two reasons. First, the key structures on a chip are not made of plastic, which melts at temperatures close to the boiling point of water, but from semiconductors and metals, which have much higher melting points.
Heating the chip to such temperatures would melt not just the structures, but nearly everything else on the chip. Secondly, the melting process would widen the structures and round off their top and side surfaces, all of which would be detrimental to the chip.
Chou's team overcame the first obstacle by using a light pulse from so-called excimer laser, similar to those used in laser eye surgery, because it heats only a very thin surface layer of a material and causes no damage to the structures underneath. The researchers carefully designed the pulse so that it would melt only semiconductor and metal structures, and not damage other parts of the chip.
The structures need to be melted for only a fraction of a millionth of a second, because molten metal and semiconductors can flow as easily as water and have high surface tension, which allows them to change shapes very quickly.
To overcome the second obstacle, Chou's team placed a plate on top of the melting structures to guide the flow of liquid. The plate prevents a molten structure from widening, and keeps its top flat and sides vertical, Chou said. In one experiment, it made the edges of 70 nanometer-wide chromium lines more than five times smoother. The resulting line smoothness was far more precise than what semiconductor researchers believe to be attainable with existing technology.
The conventional approach to fixing chip defects is to measure the exact shape of each defect, and provide a correction precisely tailored to it -- a slow and expensive process, Chou said. On the contrary, Chou's guided melting process fixes all defects on a chip in a single quick and inexpensive step.
"Regardless of the shape of each defect, it always gets fixed precisely and with no need for individual shape measurement or tailored correction," Chou said.
Next, Chou's group plans to demonstrate this technique on large (8-inch) wafers. Several leading semiconductor manufacturers have expressed keen interest in the technique, Chou said.
The study appears in the May 4 issue of Nature Nanotechnology.