May 21, 2009
Degradation study: granular metal nanostructures
Electron-beam-induced deposition (EBID) is a maskless lithography technique used for mask repair, circuit editing and sensor applications. For these applications long-term stability of the electrical characteristics of the EBID structures is crucial. In situ electrical conductivity measurements provide information about the electrical transport properties and the chemical and physical stability of granular metal nanostructures. During exposure to air the electrical conductivity of the granular metal degrades with a rate strongly dependent on the metal content.
In a recent study, W-based granular metals have been prepared by EBID with the aim of investigating the electrical transport properties during the growth process and the exposure to air. The deposits consist of metallic nanoparticles embedded in an insulating matrix. The electrical conductivity of the granular metal is a function of the metal content of the deposit, which depends on the beam parameters used during the deposition. In the investigation the authors found an increase of the electrical conductivity during the growth process, which they attributed to the growth of the metal particle's diameter. After the deposition the microscope is vented to study the degradation of the electrical conductivity during exposure to air. As shown in the figure below, the conductivity decreases with time at a rate that depends on the material's composition.
In particular, the lower the metal content the faster the decrease. The degradation is thought to be due to the effect of oxidation. Moreover, the degradation rate is linked to the density of the deposit. In view of the possible utilization of EBID deposits for applications, the composition of the granular metal has to be carefully chosen to minimize the ageing effect. Therefore, granular metals from different gas precursors employed in the EBID process will be investigated. Finally, covering the deposit with a protective SiOx layer such as the one that can be obtained from the precursor tetramethylsilane (TMS) – currently under investigation – may prevent long timescale degradation of the electrical conductivity.
The researchers presented their work in Nanotechnology.
About the author
Dr Fabrizio Porrati is a postoctoral researcher in the group Thin films and Nanostructures at the Institute of Physics, Frankfurt University. Roland Sachser joined the group as a diploma student. Prof. Michael Huth is the head of the group and he is also vice-dean of the Faculty of Physics at Frankfurt University. The work was financially supported by the NanoNetzwerkHessen (NNH) and by the Bundesministerium für Bildung und Forschung (BMBF) under grant #0312031C.