NIST Nanomechanical Properties Group researchers have developed a method to measure the mechanical and electrical properties of ultra-thin organic insulating films on metallic surfaces using conducting-probe atomic force microscopy (CPAFM). These films, whether adventitious or formed deliberately via self-assembled monolayers, have a significant impact on the electrical behavior of nano-scale metallic contacts, and thus play a key role in the performance and reliability of micro- and nano- electromechanical systems (MEMS and NEMS). Organic contaminants, such as hydrocarbons, are known to rapidly adhere to otherwise “clean” metallic surfaces exposed to ambient conditions. Hence, in order to extend the usefulness of MEMS and NEMS devices, designers require knowledge of the effects of adventitious films from the environment on device performance, as hermetic vacuum packaging used to prevent environmental exposure is often complex and expensive.
Early studies revealed the basic features of metal-insulator-metal nano-scale contacts, such as conductance values below the quantum limit and contact-load dependent non-linear current-voltage behavior, but interpretation of results to determine properties of the insulating layers was made difficult by coupling of mechanical and electrical behavior at the nano scale: Adhesive surface forces significantly modify the contact geometry from that at the macro scale and as the thickness of the insulating layer decreases to approach the Fermi wavelength of the conducting electrons, as is often the case for ultra-thin organic contamination layers, uniform tunneling barrier approximations become invalid. Nanomechanical Properties Group researchers developed a solution to this measurement problem by employing CPAFM in an ultra-high vacuum environment with contact electrodes consisting of an atomically smooth Au (111) surface and a Si cantilever probe tip coated with 60 nm of Co and 20 nm of Cr. At various points in the probe-surface contact cycle, probe displacement was halted and current-voltage measurements were performed. Using a contact mechanics model that self-consistently handles adhesive forces and deformations and a parabolic tunneling model suitable for thin insulating layers, the researchers were able to extract the contact area, electrical barrier height, and barrier thickness as a function of applied contact load.
Interestingly enough, the results suggested the presence of two insulating layers: a Cr oxide layer on the CPAFM tip and a contaminant layer on the gold surface. This technique is currently being used to examine the mechanical and electrical properties of alkanethiol self-assembled monolayers, which have been shown to provide many benefits in the performance of MEMS and NEMS, including dramatic reductions in adhesion and enhanced lubrication.
The test method was published: “Mechanical and electrical coupling at metal-insulator-metal nano-scale contacts,” D.-I. Kim, N. Pradeep, F.W. DelRio, and R.F. Cook, Applied Physics Letters 93, 203102 (2008) and “Elastic, adhesive, and charge transport properties of a metal-molecule-metal junction: the role of molecular orientation, order, and coverage,” F.W. DelRio, K.L. Steffens, C. Jaye, D.A. Fischer, and R.F. Cook, Langmuir 26 (2010) 1688-1699.
Contact: Frank W. DelRio, frank.delrio@nist.gov, (301) 975-8999.
