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February 09, 2012

10,000 times more wear resistant nanomanufactured ultrasharp silicon carbide tip

Scientists at the University of Pennsylvania, the University of Wisconsin-Madison and IBM Research - Zurich have fabricated an ultrasharp silicon carbide tip possessing such high strength that it is thousands of times more wear-resistant at the nanoscale than previous designs. The new tip, which is 100,000 times smaller than the tip of a pencil, represents an important step towards nanomanufacturing for applications, including bio sensors for healthcare and the environment.

"The dream tip material for thermomechanical nanofabrication should have a high hardness, temperature stability, chemical inertness, and high thermal conductivity," said Dr. Mark Lantz, manager in storage research at IBM Research - Zurich. "With this novel tip we continue to deliver on IBM's vision of a smarter, instrumented world with microscopic sensors monitoring everything from water pollution to patient care."

Extending their previous successful collaboration, scientists at the University of Pennsylvania, the University of Wisconsin-Madison and IBM Research - Zurich have developed a new, resistant nano-sized tip that wears away at the rate of less than one atom per millimeter of sliding on a substrate of silicon dioxide. This is much lower than the wear rate of conventional silicon tips and its hardness is 100 times greater than that of the previously state-of-the-art silicon oxide-doped diamond-like carbon tips developed by the same collaboration last year.

Scientists at the University of Pennsylvania, the University of Wisconsin-Madison and IBM Research - Zurich have fabricated an ultrasharp silicon carbide tip possessing such high strength that it is thousands of times more wear-resistant at the nanoscale than previous designs. The new tip, which is 100,000 times smaller than the tip of a pencil, represents an important step towards nanomanufacturing for applications, including bio sensors for healthcare and the environment.

Advanced Functional Materials - Wear-Resistant Nanoscale Silicon Carbide Tips for Scanning Probe Applications




The search for hard materials to extend the working life of sharp tools is an age-old problem. In recent history, sharp tools must also often withstand high temperatures and harsh chemical environments. Nanotechnology extends this quest to tools such as scanning probe tips that must be sharp on the nanoscale, but still very physically robust. Unfortunately, this combination is inherently contradictory, as mechanically strong, chemically inert materials tend to be difficult to fabricate with nanoscale fidelity. Here a novel process is described, whereby the surfaces of pre-existing, nanoscale Si tips are exposed to carbon ions and then annealed, to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance that is orders of magnitude greater than that of conventional silicon tips and at least 100-fold higher than that of monolithic, SiO-doped diamond-like-carbon (DLC) tips. The wear is well-described by an atom-by-atom wear model, from which kinetic parameters are extracted that enable the prediction of the long-time scale reliability of the tips.




A novel process is described whereby the surfaces of nanoscale Si tips are exposed to carbon ions and then annealed to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance orders of magnitude greater than conventional silicon tips and at least 100-fold higher than monolithic, SiO-doped diamond-like carbon (DLC) tips.

"Compared to our previous work in silicon, the new carbide tip can slide on a silicon dioxide surface about 10,000 times farther before the same wear volume is reached and 300 times farther than our previous diamond-like carbon tip. This is a significant achievement that will make nanomanufacturing both practical and affordable," said Prof. Robert W. Carpick, University of Pennsylvania.

To create the new tip, scientists developed a process whereby the surfaces of nanoscale silicon tips are exposed to carbon ions and then annealed so that a strong silicon carbide layer is formed, but the nanoscale sharpness of the original silicon tip is maintained. Although silicon carbide has long been known as an ideal candidate material for such tips, the unique carbon implantation and annealing process made it possible to harden the surface while maintaining the original shape and ensuring strong adhesion between the hardened surface of the tip and the underlying material—similar to how steel is tempered to make it harder.

Consisting primarily of carbon and silicon, the tip is sharpened to a nano-sized apex and integrated on the end of a silicon microcantilever for use in atomic force microscopy. The importance of the development lies not only in its ability to maintain the sharpness of the tip and its resistance to wear, but also in its endurance when sliding against a hard substrate such as silicon dioxide. Because silicon—used in almost all integrated circuit devices—oxidizes in the atmosphere, forming a thin layer of its oxide, this system is among the most relevant for emerging applications in nanolithography and nanomanufacturing applications.

More specifically, scientists hope that the new tip can be used to fabricate bio sensors, for example for managing glucose levels in diabetic patients or monitoring pollution levels in water.

Probe-based technologies are expected to play a predominant role in many such technologies. However, poor wear performance of the tip materials used so far, especially when slid against silicon oxide, have previously limited their usefulness for experimental applications.

The next step for scientists is to begin testing the new tip for use in applications, starting with nanomanufacturing.

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