Portland, Ore. -- Vias on printed-circuit boards are metal-clad holes that connect component leads on the top to component leads on the bottom. Similarly, on chips, semiconductor circuitry on one layer can be connected to a second layer via a hole, etched in the interlayer dielectric, which is then filled with metal. Now Purdue University engineers have demonstrated a semiconductor fabrication technique that uses tiny, 30- to 50-nanometer-pitch arrays of vertically grown carbon nanotubes for vias be- tween layers on semiconductor chips.
The technique, devised by Timothy Fisher and Timothy Sands at Purdue's Birck Nanotechnology Center, works by virtue of "porous anodic alumina," which can form a network of evenly spaced pores in an aluminum oxide dielectric layer and then grow carbon nanotubes through the pores to form vias between layers. The technique produces both single- and double-walled carbon nanotubes.
Each pore grows only a single tube, suggesting that the vias can be strictly controlled. That would make them easier to integrate with normal CMOS circuitry.
The first step was to grow a silicon dioxide adhesion layer atop a single-crystal silicon wafer. Then a titanium layer was added, topped with an aluminum layer, an iron layer and another aluminum layer. Anodization oxidized the top aluminum layer, with its parameters set for pores spaced at 30 to 50 nm. An electric field kept the pores aligned in evenly spaced rows.
Since carbon nanotubes arise from iron seeds, all they needed to grow was a mixture of hydrogen and methane gas flowed over the holes, inducing synthesis from the catalyst layer by plasma-enhanced chemical-vapor deposition. Microwaves energized the process, breaking down the methane into pure carbon, with the iron layer acting as the catalyst to prompt carbon nanotube growth in the pores. Once begun, the growing process continued as long as methane was present. Fisher and Sands and their group reported that it took only a few minutes to grow single nanotubes in each pore between the layers.
For the future, the group plans to control the type of nanotubes grown--single-walled, double-walled, metallic or semiconducting. They also hope to characterize the conditions required to grow each type.
Contributing to the work were Eric Stach, an associate professor of materials engineering; Dmitri Zakharov, a staff scientist at the Birck Nanotechnology Center; postdoctoral research associate Placidus Amama; and graduate students Matthew R. Maschmann and Aaron D. Franklin.
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