Silicon ‘Velcro’ exerts needle-like grip

By etching silicon in such a way as to form barbed needles on its surface, it is possible to make a press joint to polymer that is stronger than the parent materials. Applications are seen in fluid connections for use in the laboratory, as well as micro electronic and micro mechanical structures. This development of ‘Black silicon’ to polymer bonding is the brainchild of Mike Stubenrauch, a researcher at the Technical University of Ilmenau in Germany. The silicon is etched in the areas to be bonded, in such a way as to form needles 1-25 microns long and with tip diameters of 100-500nm. There are 2 million needles per square mm. By employing ‘gas chopping’ – ie, cyclic changes between etch and passivation steps – it is possible to make the needles barbed. The silicon and polymer-based parts are then aligned and pressed together with a pressure of 2-5 MPa – depending on the viscosity of the material – which drives the barbed needles into the polymer surface. The force has to be carefully applied, so that it is properly perpendicular to the silicon surface. Shear forces cause the needles to disintegrate. “Black silicon is not a new idea,” admits Christoph Kremin, also from the University of Ilmenau. “But our needles are longer than anyone else’s, so we can get stronger adhesion. We have achieved retention forces of up to 2020 N/sq cm.” A sample of a small plastic syringe with a 4mm diameter Luer connector end, bonded to a piece of silicon with a deep etched ring of needles, proved the material’s effectiveness. It was impossible to prise the joint apart, because the silicon outside the ring bond always breaks first. “It is possible to pull the silicon-to-silicon bonds apart and then re-attach them,” adds Kremin. “We have done it. But every re-attachment causes some destruction of the needles. You can re-attach the needles at least five times.” But this does not work for the polymer-to-silicon bond, which only works once. And while the bonding interface is not described as hermetic, the developers have successfully made connections that have proved to be watertight up to 0.7MPa (7 bar). Bonding to ceramics A development of the technique also allows it to be used to form strong bonds to ceramics. The team has used a Low Temperature Co-fired Ceramic (LTCC) foil called BGK, from the Hermsdorfer Institut fur Technische Keramik. The polymer-based foil is laminated to the silicon in the same way as an unfilled polymer. The temperature is about 70ºC and the bonding pressure 5MPa. The ceramic foil-silicon assembly is then heated to 850ºC under uniaxial pressure. The polymer binder is evaporated and a solid ceramic formed. A penetration depth of 2-3 microns is required for high retention forces. More than 1000N/sq cm has been reported. Potential applications include fluidic interconnects for use in the laboratory, non-hermetic sealing of silicon chips and devices, and other polymer silicon components that need additive-free connections and high bonding forces, such as microsystems in biomedical environments. The ceramic-to-silicon bonding process could also be very useful for making integrated small packaging solutions for silicon-based functional elements and three-dimensional structures with fluidic and electrical interconnections. Pointers * It is possible to form strong, dry bonds between silicon and polymers by deeply etching the silicon to form barbed needles and pressing the two materials together * It is also possible to form quite strong bonds with ceramics by pressing needle-covered silicon against a polymer filled with ceramic particles and evaporating away the polymer *Retention forces of 2000N/sq cm for silicon to polymer and 1000N/sq cm for silicon to ceramic are possible