Long-term Adhesion Studies of Polyimide to Inorganic and Metallic Layers
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Long-term Adhesion Studies of Polyimide to Inorganic and Metallic Layers Juan S. Ordonez1, Christian Boehler1, Martin Schuettler1, Thomas Stieglitz1,2,3 1. Department of Microsystems Engineering - Lab. for Biomedical Microtechnology, University of Freiburg, Freiburg, Germany. 2. Bernstein Center Freiburg, University of Freiburg, Freiburg, Germany. 3. Freiburg Institute for Advanced Studies-FRIAS, University of Freiburg, Freiburg, Germany ABSTRACT Delamination between thin-film metal and substrate is a major cause of failure in polyimide based neural microelectrode arrays. Chemical adhesion is the only way to establish a long-term bond that will allow two materials to stick to each other even in a wet environment, given that the materials do not deteriorate in the presence of water. This study assesses, by means of peel and shear tests, a long-term quantitative and comparative study of the adhesion of polyimide to various metallic and other inorganic layers of interest. Polyimide (BPDA-PPD) was cured on the layers, which involve platinum, gold and tungsten-titanium as commonly used implant metals and diamond-like carbon (DLC), silicon carbide (SiC), silicon dioxide (SiO2) and silicone nitride (SiN) as potential adhesion promoters to be used later as intermediate layers between metal and polyimide. The adhesion was observed over one year under accelerated-aging conditions by storing the specimens in 60°C saline (corresponding to 40000 hrs at 37°C). Only silicon carbide and amorphous carbon showed almost unaffected adhesion to polyimide over the testing period. No water intrusion at the interface was observed and the strong bond is almost fully maintained. INTRODUCTION Flexible electronics or neural interfaces are examples for the use of a polymer material as a carrier substrate for delicate metallic wiring. All applications rely on the adhesion of the metallic layer to the polymer substrate in order to succeed in application. Not only the dimensions of the structures make these devices extremely delicate, but also the plasma deposition processes do not provide a though crystalline structure as given in bulk materials [1]. Any fractional force will break these 300 nm thin metallic wires if adhesion to the substrate is not given. The adhesion strength defines indirectly the extent to which a load on the system will be dissipated and shared by the materials. The application inside the body -a wet, ionic environment- makes maintaining an interlayer bond over decades a challenging task. Water intrusion at an interface will affect physical adhesion (polar attractions, mechanical interlocking). Establishing a long-term bond allowing two materials to stick to each other even in a wet environment is only possible by chemical adhesion. Different approaches have been investigated to promote adhesion. So far, they have shown only moderate improvements but no real proof of the long-term stability has been presented in literature worldwide. Oxygen plasma has been successfully applied to roughen the surface and achieve a better inte
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