Formation of High-Resistivity Silver-Silicon Dioxide Composite Thin Films Using Sputter Deposition
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Formation of High-Resistivity Silver-Silicon Dioxide Composite Thin Films Using Sputter Deposition. Ann Marie Shover, James M.E. Harper, Nicholas S. Dellas, Warren J. MoberlyChan1 Department of Physics, University of New Hampshire Durham, New Hampshire 03824, USA 1 Center for Imaging and Mesoscale Structures, Harvard University Cambridge, MA 02138, USA ABSTRACT Composite Ag-SiO2 thin films were deposited to examine the stability of materials with high resistivity above 5000 µΩ-cm. We found that the resistivity increases exponentially with SiO2 volume fractions larger than 0.50 which is consistent with a tunneling conductivity mechanism. In order to obtain a broad composition range, these films were deposited on a stationary substrate placed above Ag and SiO2 sputtering sources. This configuration allowed compositions ranging from 0.8 to 56.3% SiO2 to be deposited on the same sample. Resistance measurements were made using a four-point probe and a profilometer was used to measure thickness. Predicted values of thickness and composition were obtained by calibrating deposition rates from the separate sources, and were verified using Rutherford Backscattering Spectroscopy. Electron microscopy analysis revealed Ag agglomerating on the surface of the film. Because of the high mobility of Ag, the films should be capped to prevent Ag agglomeration. The results of this research demonstrate that high-resistivity thin films can be grown using Ag-SiO2 composites.
INTRODUCTION The aim of this research project was to determine whether composition could be consistently controlled for co-deposited metal-insulator thin films in order to achieve high resistivities. These thin films have utility in a variety of space sensor applications. In particular, cosmic ray detectors require resistivities greater than 20,000 Ω/square and this research indicates that such values can be achieved with our composition control techniques. In addition, the deposition of multi-component thin films will be used in the formation of nanoscale templates and in molecular sensors that depend on obtaining specific microstructures and compositions. To date, a number of metal-insulator combinations including Ni-SiO2 [1] and W-Al2O3 [2] have been grown to examine percolation and tunneling properties for these films. Resistivities for Ag-SiO2 thin films have been reported in the literature and the data suggests critical values for percolation between 0.4-0.6 volume fraction SiO2[3,4]. EXPERIMENT Our setup consists of a custom-designed vacuum chamber (Figure 1) with an approximate 1.0 m volume. The chamber contains two three-inch diameter magnetron sputtering sources. The substrates were strips of polished silicon approximately 2 x 12 cm in dimension, vitreous carbon and TEM windows. These were inserted into the chamber through a load-lock. Base pressures were typically 1-2 x10-8 Torr. The deposition pressures were 1.0 mTorr and 2.0 mTorr Argon. 3
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Deposition rates at the center of the substrate, using DC power for Ag and RF power for SiO2, are
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