Growing a Forest of Nanotubes on Silicon Chips

Engineers at Purdue University's Birck Nanotechnology Center have grown forests of tiny carbon cylinders called carbon nanotubes onto the surfaces of silicon computer chips.

Researchers made this breakthrough in an effort, which is funded by NASA, to develop new types of thermal interface materials that conduct more efficiently than conventional materials. The aim is to enhance the flow of heat at a critical point where the chips connect to cooling devices calledheat sinks.

Heat sinks (also heatsinks) are metal structures that usually contain an array of tiny metal fins to increase surface contact with the air. These fins thereby also improve heat dissipation.

The objective of the research is to improve overall performance and help meet cooling needs of future computer chips. Innovations in computer technology mean that future chips will produce more heat than current microprocessors do.

The carbon nanotubes that are grown on the silicon chips are sandwiched between the chips and the metal heat sinks (structures with tiny fins providing air contact). The nanotubes fill the gaps and irregularities between the chip and the metal heat sink. This contact between the carbon nanotubes and the heat sink enhances heat flow between the two.

According to doctoral student Baratunde A. Cola, who is a co-author on the Purdue research paper, the method developed by the Purdue researcher team enables them to create a thermal interface that conforms to a heat sink's uneven surface. Conventional thermal interface materials include greases, waxes and a foil made of a metal called indium. Having an interface that conforms to the heat sink allows for the conduction of heat with less resistance than comparable interface materials currently in use by industry.

The carpet-like growth of the carbon nanotubes has been shown to outperform the conventional thermal interface materials that interface between the heat sink and the electronic component's hot surface.

Better thermal interface materials are needed either to test computer chips in manufacturing or to keep chips cooler during operation in commercial products. All of the standard interface materials have drawbacks. The greases don't last many cycles of repeatedly testing chips on the assembly line. The indium foil doesn't make good enough contact for optimum heat transfer, according to co-author and Purdue professor of mechanical engineering Timothy S. Fisher.

To grow the forests of carbon nanotubes, the Purdue researchers created templates from branching molecules called dendrimers, forming these templates on a silicon surface. In other words, a pattern used as a guide and made from a polymer whose atoms are arranged in many branches along a central spine of carbon atoms was formed on a silicon surface.

Then, metal catalyst particles that are needed to grow the nanotubes were deposited inside the cavities between the dendrimer branches:within the spaces of the template. The catalyst particles are made of "transition metals" such as iron, cobalt, nickel or palladium. Because the catalyst particles are about 10 nanometers in diameter, they allow the formation of tubes of similar diameter. Researchers then applied heat to the silicon chip that was holding the template and catalyst. The polymer template was burned away. That left behind only the metal catalyst particles.

Next, the engineers placed the catalyst particle-laden silicon chip inside a special chamber and exposed it to methane gas. Microwave energy was then applied to break down the methane. Methane contains carbon and so, once broken down by microwave energy, it became the "seed" carbon for growing the nanotubes. The catalyst particles then prompted the carbon nanotubes to assemble from carbon that originated in the methane. The tubes, which were guided in their formation by the catalysts implanted in the previously existing polymer template, then grew vertically from the surface of the silicon chip.

Because the branching dendrimers have a uniform composition and structure, the researchers were able to control the distribution and density of catalyst particles that determined the areas of vertical growth of the carbon nanotubes.

"The dendrimer is a vehicle to deliver the cargo of catalyst particles, making it possible for us to seed the carbon nanotube growth right on the substrate...We are able to control the particle size - what ultimately determines the diameters of the tubes - and we also have control over the density, or the thickness of this forest of nanotubes. The density, quality and diameter are key parameters in controlling the heat-transfer properties," said Placidus B. Amama, a postdoctoral research associate at the Birck Nanotechnology Center in Purdue's Discovery Park.

Researchers usually produce carbon nanotubes separately and then attach them to the silicon chips or mix them with a polymer and then apply them as a paste.

"Our direct growth approach, however, addresses the critical heat-flow path, which is between the chip surface and the nanotubes themselves," Fisher said. "Without this direct connection, the thermal performance suffers greatly."

"In a personal computer, laptop and portable electronics, the better your thermal interface material, the smaller the heat sink and overall chip-cooling systems have to be," Cola said.

Findings were detailed in a research paper that appeared in September's issue of the journal Nanotechnology. The paper was written by Amama; Cola; Timothy D. Sands, director of the Birck Nanotechnology Center and the Basil S. Turner Professor of Materials Engineering and Electrical and Computer Engineering; and Xianfan Xu and Timothy S. Fisher, both professors of mechanical engineering.

"Nanotube forests grown on silicon chips for future computers, electronics," Birck Nanotechnology Center in Purdue University's Discovery Park.