A new process using lasers instead of high temperatures to remove hydrogen from silicon during the chip-manufacturing process could lead to faster semiconductors, by replacing the current technique, which often causes damage to silicon that inhibits chip speed.
Today, semiconductors are manufactured by layering silicon on a wafer, one “sheet” at a time. During this process, oxygen, which is a byproduct, can collect between the silicon layers – which ruins the chip. To prevent that from happening, hydrogen is added to the silicon as a protective coating. While it solves the oxygen issue, the step has its own, albeit lesser, drawback: before the next layer of silicon can be added, the hydrogen must be removed, in a process that currently requires heating the chip to around 800 degrees Celsius. This heating creates defects in the silicon that keep chips from performing at their optimal speeds.
This new laser process, which can target and selectively remove molecules without heating the silicon, could replace the heating step, says Norman Tolk, physics professor at Vanderbilt University, and one of the researchers on the project. “The more you heat [silicon], the more you put it in a hostile environment,” he says. Ideally, the chip-making process should be done with temperatures that are as low as possible, he says.
In a hydrogen-silicon bond, the energy required to break the bond corresponds to infrared light with a wavelength of 4.8 micrometers. The researchers adjusted their extremely powerful laser (called a “free electron laser”) to emit a beam at this wavelength, and bathed the silicon-hydrogen bonds with the light. The laser’s energy caused the bonded atoms to bounce back and forth, as if on a spring, until the vibrations grew large enough to break the bonds.
In a second part of the experiment, the researchers tested the ability of the laser to selectively remove hydrogen from the surface of the silicon when other types of atoms were present, in an effort to broaden the implication of their findings. When they included deuterium atoms (a heavy form of hydrogen) on the silicon, the laser stripped away only the hydrogen, leaving the deuterium behind. (Since the deuterium atoms are heavier, they don’t vibrate at the same frequency as hydrogen and are therefore invisible to the 4.8 micrometer wavelength light, Tolk explains.)
One of the next steps for the researchers, says Leonard Feldman, physics professor at Vanderbilt and a researcher on the team, is to test their bond-breaking technique on the type of silicon that has a crystal structure most commonly used in the semiconductor industry. (In the experiment, the researchers used silicon with a crystal structure that had been studied thoroughly in terms of silicon-hydrogen bond formation and breaking.) Also, he says, in order to get a broader understanding of the physical processes involved in breaking hydrogen bonds, the researchers will test materials other than silicon, such as diamond, which “tends to behave like silicon” in terms of hydrogen bonding, Tolk says.