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Platinum Metals Rev., 1996, 40, (3), 116

Palladium Colloid Catalyst Used in Microcontact Printing

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There are several methods in current use for transferring very fine patterns onto substrates for printed electronic circuitry. These all involve the selective metallisation of the area to be treated and use various techniques, such as photolithography or electroless plating. However, scientists at Harvard University have now announced a new method of electroless deposition, which they have demonstrated with copper but suggest could also be used for the deposition of other metals (P. C. Hidber, W. Helbig, E. Kim and G. M. Whitesides, “Microcontact Printing of Palladium Colloids: Micron-Scale Patterning by Electroless Deposition of Copper”, Langmuiri, 1996, 12, (5), 1375–1380).

Their new strategy involves the manual transfer of a palladium colloid catalyst onto a substrate surface by microcontact printing (μCP); this uses a patterned elastomer stamp made from poly(dimethylsiloxane). The stamp is previously dipped into the palladium colloid, which has been stabilised with tetraalkylammonium bromides. This is followed by the electroless deposition of copper which proceeds by immersion of the substrate in a copper plating bath, and occurs only at the regions coated with the palladium colloid, where a catalytic reaction occurs.

Copper lines of micron and submicron widths, having edge resolution of 100 nm, were produced on a variety of substrates, including glass, silicon with a silicon dioxide layer, and polymers. Both flat and curved surfaces can be plated without loss of resolution. In addition, free-standing, flexible structures can be produced by dissolving the substrate when the metal film reaches the required thickness or by allowing the internal stress in the electroless copper layer to exceed the adhesion strength, when delam-ination occurs.

While films of approximately uniform thickness can be produced by this method, ways of obtaining structures which have different layer thicknesses have also been developed.

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