Synthesising graphene in silicon microelectronics

Korean researchers have developed an easy and microelectronics-compatible method to grow 4″ diameter, high-quality, multilayer graphene on desired silicon substrates — an important step for harnessing graphene in commercial silicon microelectronics.

The method is based on an ion implantation technique, a process in which ions are accelerated under an electrical field and smashed into a semiconductor. The impacting ions change the physical, chemical or electrical properties of the semiconductor.

"For integrating graphene into advanced silicon microelectronics, large-area graphene free of wrinkles, tears and residues must be deposited on silicon wafers at low temperatures, which cannot be achieved with conventional graphene synthesis techniques as they often require high temperatures," said Jihyun Kim, the team leader and a professor in the Department of Chemical and Biological Engineering at Korea University, in Seoul.

"Our work shows that the carbon ion implantation technique has great potential for the direct synthesis of wafer-scale graphene for integrated circuit technologies."

In a paper published this week in the journal Applied Physics Letters, from AIP Publishing, the researchers describe their work, which takes graphene a step closer to commercial applications in silicon microelectronics.

"In silicon microelectronics, graphene is a potential contact electrode and an interconnection material linking semiconductor devices to form the desired electrical circuits," said Kim.

"This renders high-processing temperature undesirable, as temperature-induced damage, strains, metal spiking and unintentional diffusion of dopants may occur."

Thus, although the conventional graphene fabrication method of chemical vapour deposition is widely used for the large-area synthesis of graphene on copper and nickel films, the method is not suitable for silicon microelectronics as chemical vapour deposition would require a high growth temperature above 1000°C and a subsequent transfer process of the graphene from the metallic film to the silicon.

"The transferred graphene on the target substrate often contains cracks, wrinkles and contaminants," said Kim.

"Thus, we are motivated to develop a transfer-free method to directly synthesise high-quality, multilayer graphene in silicon microelectronics."

Kim's method relies on ion implantation, a microelectronics-compatible technique normally used to introduce impurities into semiconductors. In the process, carbon ions were accelerated under an electrical field and bombarded onto a layered surface made of nickel, silicon dioxide and silicon at the temperature of 500°C. The nickel layer, with high carbon solubility, is used as a catalyst for graphene synthesis. The process is then followed by high-temperature activation annealing (about 600 to 900°C) to form a honeycomb lattice of carbon atoms, a typical microscopic structure of graphene.

Kim explained that the activation annealing temperature could be lowered by performing the ion implantation at an elevated temperature. Kim and his colleagues then systematically studied the effects of the annealing conditions on the synthesis of high-quality, multilayer graphene by varying the ambient pressure, ambient gas, temperature and time during the treatment.

According to Kim, the ion implantation technique also offers finer control on the final structure of the product than other fabrication methods as the graphene layer thickness can be precisely determined by controlling the dose of carbon ion implantation.

"Our synthesis method is controllable and scalable, allowing us to obtain graphene as large as the size of the silicon wafer [over 300 mm in diameter]," Kim said.

The researchers' next step is to further lower the temperature in the synthesis process and to control the thickness of the graphene for manufacturing production.

Image caption: This is a wafer-scale (4″ in diameter) synthesis of multilayer graphene using high-temperature carbon ion implantation on nickel/SiO₂/silicon. Credit: J.Kim/Korea University, Korea.