Micro-scale metallization of high aspect-ratio Cu and Au lines on flexible polyimide substrate by electroplating using SU-8 photoresist mask

Abstract

In order to fabricate flexible microelectronic devices, fabrication of metallization lines and metal electrodes on the flexible substrate is essential. Cu lines are often used as interconnect lines in electronic devices and Au as microelectrodes in organic transistors and bioelectronics devices due to its good electrochemical stability and biocompatibility. For minimizing the size of device, the realization of metallization lines and microelectrodes with the scale of a few micrometers on the flexible substrate is very important. In this work, micro-scale metallization lines of Cu and Au were fabricated on the flexible polyimide (PI) substrate by electroplating using the patterned mask of a negative-tone SU-8 photoresist. Surface of PI substrate was treated by O 2 inductively coupled plasma for improvement of the adhesion strength between Cr layer and PI and in situ sputter-deposition of 100-nm thick Cu seed layers on the sputter-deposited 50-nm thick Cr adhesion layer was followed. Electroplating of high aspect-ratio Cu and Au lines using a sulfuric acid and a noncyanide solution with the patterned SU-8 mask, respectively, removal of SU-8, and selective wet etch of Cr adhesion and Cu seed layers were carried out. Micro-scale Au electrode lines were successfully fabricated on the PI substrate. Micro-scale gap-filled Cu lines with spin-coated polyimide on the PI substrate with the thickness of 6–12 μm and the aspect ratio of 1–3 were successfully fabricated.