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Using ion implantation to embed conductive layers in polymer
Polymer use, so far hampered by poor water wettability and the fact that the material does not conduct electricity, will be boosted by a new fabrication technique.
12 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200704.0651
Polymers are used in various technologies and for a wide range of applications because of their excellent physical and chemical properties coupled with their low cost. Some polymers have properties that are particularly desirable for biomedical applications: these include a density comparable to human tissue, good fracture toughness, resistance to corrosion, and ease of forming by molding or machining.1
However, the use of polymers is still limited because of their poor wettability and adhesion, inherent softness, and unexpected dielectric properties.
To enhance the surface properties of polymers, a variety of modification techniques have been evaluated including plasma, radiation, and ion beam treatment. The technique of producing films by combining plasma immersion ion implantation (PIII) and cathodic arc sources has attracted considerable interest over the last decade because it produces high-quality coatings2-5
and makes possible the doping of materials6,7
at room temperature. First introduced in the 1980s, PIII also has several other advantages: these include the ability to treat irregular surfaces; providing superior adhesion to substrates;6,8
and a high implantation rate.
However, there are problems inherent to the application of PIII to non-conducting materials such as polymers, and these are due to surface charging. To overcome these we sputtered a thin layer of gold before PIII is applied to the polymer substrate. The result was a controllable implantation depth and stronger adhesion between the metal-polymer interfaces. The extent of implantation depth can be correlated to tribological properties, electrical conductivity, and Raman spectroscopy. While conductive atomic force microscopy (AFM) confirmed the conductivity of the embedded layer, the future applications, difficulties, and limitations of this technique for fabrication of conductive embedded layer in polymers are still of interest.
A comparison of differences in Raman spectra obtained from pristine to gold-sputtered and PIII polymer surfaces are as shown in Figure 1
. The Raman spectra in Figure 1
c) show the return of polymer to the surface or, in fact, that the gold layer has been sputtered away by the incoming ions from the plasma. A simple linear scratch test shows the critical load increased more than 100% from 2.1N to approximately 5N. Hardness and indentation tests were also carried out, but were inconclusive due to the substrate effect and dislocation in the sputtered and PIII layers.
Figure 1. Surface Raman spectra: a) pristine plain polymer, b) gold sputtered polymer surface, and c) plasma-modified gold-sputtered-polymer surface.
To show that there is some embedded conductivity in the PIII-modified films, we increased the bias voltage applied to a low conductivity film and observed the changes, as show in Figure 2
. The left-hand side is the surface topography of the PIII-modified polymer while the right-hand side shows the conductivity of the same region. The brightness of the spots in the images indicates the current intensity obtained. As the bias voltage increased from 1V to 3V, we observed high conductivity at the regions of non-conductivity. This indicates that some electrons have tunnelled through the surface layer or, in other words, there exists some electrical conductivity embedded in the polymer.
Figure 2. Topographic images of PIII-modified polymer surfaces are shown in T1 and T2. The mapping of electrical conductivity (current flow) from the substrate to the tip of the atomic force microscope, for the same surfaces, are shown in C1 and C2. The current intensity is shown by brightness on the conductivity images.
In summary, the PIII technique can help make non-conducting polymer conduct electricity. It has been shown to increase metal-polymer adhesion and also electrical conductivities both at the surface and also underneath: this was demonstrated by the observation of electron tunneling when bias voltages were increased. This fabrication technique should open up more exciting applications for non-conducting polymers, including those involving embedded conductive-layer open paths for future electronic devices.
P. C. T. Ha, Z. J. Han
Nanoelectronics Laboratory, School of Electrical and Electronic Engineering,
Nanyang Technological University
Dr. Peter C. T. Ha has two PhDs: one from the City University of Hong Kong and another from the University of Sydney. He also holds an Australian Endeavor Cheung Kong Award for outgoing international networking in Asia. He is currently a research fellow at Nanyang Technological University.
B. K. Tay
Nanyang Technological University
1. D. R. McKenzie, K. Newton-McGee, P. Ruch, M. M. Bilek, and B. K. Gan, "Modification
of polymers by plasma-based ion implantation for biomedical applications,"
Surface and Coatings Tech. 186(1-2), pp. 239-244, 2004.
2. D. R. McKenzie, R. N. Tarrant, M. M. M. Bilek, T. Ha, J. Zou, W. E. McBride, D.
J. H. Cockayne, N. Fujisawa, M. V. Swain, and N. L. James, "Multilayered carbon
films for tribological applications," Diamond and Related Materials 12(2), pp. 178-
184, 2003. PII: S0925-9635Z 03. 00020-7.
3. D. Sheeja, B. K. Tay, S. M. Krishnan, and L. N. Nung, "Tribological characterization
of diamond-like carbon (dlc) coatings sliding against dlc coatings," Diamond
and Related Materials 12(8), pp. 1389-1395, 2003.
4. Z. J. Han, P. C. T. Ha, and B. K. Tay, "Xps studies on aluminum ions modified
polyimide with piii technique," J. App. Phys. , pp. JR06-3004R, 2007. Accepted.
5. J. Y. Sze, B. K. Tay, C. I. Pakes, D. N. Jamieson, and S. Prawer, "Conducting ni
nanoparticles in an ion-modified polymer," J. App. Phys. 98(6), p. 066101, 2005.
6. J. Ho, R. Poon, Y. Xie, P. Ha, and P. Chu, "Anti-corrosion properties of nitrogen
and oxygen plasma-implanted nickel-titanium shape memory alloy," Solid State
Phenomena 107, pp. 111-114, 2005.
7. S. C. H. Kwok, P. C. T. Ha, D. R. McKenzie, M. M. Bilek, and P. K. Chu, "Biocompatibility
of calcium and phosphorus doped diamond-like carbon thin films
synthesized by plasma immersion ion implantation and deposition," Diamond and
Related Materials 15, pp. 893-897, 2006.
8. P. C. T. Ha, D. R. McKenzie, M. M. M. Bilek, D. Doyle, D. G. McCulloch, and
P. K. Chu, "Control of stress and delamination in single and multi-layer carbon
thin films prepared by cathodic arc and rf plasma deposition and implantation.,"
Surface and Coatings Technology , pp. 6405-6408, 2006.