Apr 16, 2009
Wafer-scale processes single out CNTs
Controlled nanoscale 3D device architectures based on vertically oriented carbon nanotubes (CNTs) are promising for many applications in electronics such as nano-electro-mechanical systems (NEMS), field emitters and sensors. However, such controlled structures are often difficult to make, especially using low-cost, wafer-scale approaches, due to stringent tube synthesis requirements.
Now, researchers at the Jet Propulsion Laboratory (JPL), California Institute of Technology, US, have combined top-down nanofabrication approaches with bottom-up tube synthesis techniques to form controlled structures comprising of just single tubes. In their paper, published recently in Nanotechnology, low-cost, wafer-scale approaches were used to form single, aligned tubes that were centred precisely and placed within a few hundred nanometres of high-aspect ratio 3D nano-electrodes.
In the past, e-beam lithography has typically been used to lithographically define catalyst islands below a few hundred nanometres in diameter. These suitably sized catalyst sites then go on to serve as a template for nucleating single tubes. However, e-beam lithography is slow and expensive, and as a result, limits the transition of nanoscale devices from the laboratory into commercial production. Instead, the JPL team has implemented deep-UV chemically amplified resists with step-and-repeat optical lithography, as a low-cost, top-down approach to define deep-submicron catalyst features. Such catalyst sites not only nucleated single tubes, but these tubes grew with unprecedented alignment accuracy within pre-existing nano-electrodes.
Although CNTs have exceptional material properties, it is still difficult to control their characteristics. This lack of control over properties is not uncommon with other bottom-up processes, where single atoms and molecules have to arrange themselves into structures that extend into the micro-scale and beyond. For example, conventional CNT growth techniques such as thermal chemical vapour deposition (CVD), generate randomly oriented tubes that form a "mat", making them undesirable for some applications. The JPL group employed plasma-enhanced (PE) CVD, where the tubes align to the electric field within the plasma as they emerge from the narrow gap between the pre-defined electrodes. The materials used for the electrodes were also compatible with the corrosive and high-temperature CNT growth environment. Such top-down and bottom-up wafer-scale approaches developed here should accelerate the integration of PECVD-grown nanotubes for a variety of applications in 3D-electronics.
About the author
Anupama B Kaul is a principal investigator at the Jet Propulsion Labs, where she is leading the development of NEMS devices for their application in extreme environment space electronics. Krikor Megerian is involved in developing fabrication processes for a wide variety of sensors and detectors, including nanotube-based devices at the JPL Microdevices Laboratory. Paul von Allmen is supervisor of the modeling group at JPL, where he is creating finite-element codes for simulating CNT-based NEMS devices. Richard L Baron is a planetary scientist at JPL and develops new instrument concepts for NASA's earth science missions. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and was funded through the internal Research and Technology Development program.