Tough New Probe Developed For Nanotechnologists

A common problem in atomic force microscopy is the deterioration ofthe tip apex as surfaces are scanned. To overcome this problem, a teamof scientists from Northwestern University and Argonne NationalLaboratory report the microfabrication of monolithicultra-nano-crystalline diamond (UNCD) cantilevers with tips exhibitingproperties similar to single-crystal diamond. Their results arepublished in the Aug. 9 issue of Small, a journal dedicated tobreakthroughs in nanoscience and engineering (http://dx.doi.org/10.1002/smll.200500028).

Diamond, the hardest known material, is probably the optimal tipmaterial for many applications. In addition to hardness, diamond isstiff, biocompatible and wear resistant. Until now, commerciallyavailable diamond AFM tips are either glued to a microcantilever (avery slow and non-scalable manufacturing approach) or made by coating asilicon tip manufactured using conventional microfabricationtechniques. Chemical vapor deposition (CVD) techniques for growing thinfilms of synthetic diamond typically do not produce single-crystalfilms, in which atoms are all oriented in a regular lattice. UNCD, amaterial discovered at Argonne in the 1990s, is the closest diamondatomic structure in which the material is organized in very smallgrains (a few nanometers in size) leading to smooth surfaces easy tomold and shape by microfabrication techniques. The similarity of UNCDto single-crystal diamond and its superiority to silicon, siliconcarbide and other micro- and nanoelectromechanical systems (MEMS andNEMS) materials, in the context of strength, toughness and wearperformance, has been established.

The standard MEMS microfabrication techniques used for the diamond tips-- an important feature of this development -- provides scalability tomassively parallel arrays of probes for high throughput.

"This technology offers tremendous potential for the large-scaleproduction of single- and two-dimensional tip arrays of doped andundoped diamond exhibiting superior wear resistance and functionality,"said team leader, Horacio D. Espinosa, professor of mechanicalengineering at Northwestern's McCormick School of Engineering andApplied Science. "The approach can be easily integrated with the AFMNanofountain Probe (NFP) recently developed by our group and, in thisway, achieve the merging of two unique technologies.

"The demonstrated low wear and writing capability of UNCD tipswith chemical inks is very promising. Moreover, the possibility ofdoping the material to make it conductive is very exciting and opens alarge number of opportunities for scientific discovery. We believe theprocessing technology will likely lead to many novel applications notonly in AFM tips but also in MEMS and NEMS."

Potential products range from single UNCD AFM tips for use incommercial AFMs to massively parallel probe systems for high-throughputscanning and nanoscale manufacture. The technology can be employed fora variety of AFM scanning modes, from regular surface scanning in airor fluids to conductive AFM. It can also be employed as ananofabrication tool. Examples include nanolithography (inorganic inkdispensing), detecting and repairing failure of micro- and nano-electronic devices, nanopatterning of biomolecules (for sequencing,synthesis and drug discovery) and scanning probe electrochemistry(scanning electrode imaging, localized electrochemical etching ordeposition of materials and nanovoltametry).

Potential markets include those industries where it is pivotal topreserve the performance of the tips or that require two-dimensionalarrays for high throughput in which the cost of manufacturing is suchthat minimum possible tip wear is paramount. Examples include themicroelectronics industry (novel random-access memories based on AFMtechnology, such as IBM's Millipede), the semiconductor industry(photomask repair) and the chemical and biological sensor industrywhere high throughput and spatial resolution are important.

Northwestern is seeking a licensing partner to commercialize themicrofabrication processes and methods to produce arrays of the device.A patent application has been filed by the University.

In addition to Espinosa, otherauthors on the Small paper are graduate student Keun-Ho Kim andresearch assistant professor Nicolaie A. Moldovan from Northwestern andpost-doctoral fellow Xingcheng Xiao and research scientists JohnCarlisle and Orlando Auciello from Argonne National Laboratory (ANL).This research has been supported in part by the National ScienceFoundation through two initiatives: Nanoscale InterdisciplinaryResearch Team Award No. CMS00304472 and Nanoscale Science andEngineering Center Award No. EEC-0118025. The ANL team effort has beensupported by the Department of Energy's Office of Science under AwardNo. W-31-109-ENG-38.