Micro-Scale Metallization of Cu and Au on Flexible Polyimide Substrate by Electroplating Using SU-8 Photoresist Mask
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Micro-Scale Metallization of Cu and Au on Flexible Polyimide Substrate by Electroplating Using SU-8 Photoresist Mask Suhyeon Cho, Soohong Kim, Nae-eung Lee* Dept. of Materials Engineering and Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon, Gyunggi-do 440-746, Korea 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 O2 inductively coupled plasma for improvement in the adhesion between the Cr layer and the PI and in-situ sputter-deposition of 100-nm-thick Cu seed layers on the sputter-deposited 50-nmthick Cr adhesion layer was followed. Electroplating of 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 spincoated polyimide on the PI substrate with the thickness of 6 ~ 12 ㎛ and the aspect ratio of 1~3 were successfully fabricated.
INTRODUCTION Polymer for microelectric applications has attracted a great deal of concern in the past few years because polymers can be applied to the manufacturing of various electronic and display devices. There have been extensive research activities on flexible electronics based on polymeric materials [1]. Flexible TFT (thin film transistor), packaging, and FPCB (flexible printed circuit board) based on all polymeric materials or partial employment of polymeric materials have been developed due to low cost and ease of fabrication [2, 3]. Cu metallization on flexible substrate has been frequently used in FPCB and advanced packaging technology. In this case, a subtractive fabrication method for Cu lines with the scale of several tens of micrometers has usually been used [4]. In this method, wet etching of Cu from the
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Cu foil or thin films on flexile substrate delineates Cu line patterns. For the fabrication of Cu lines with the width of several micrometers and a high aspect ratio, however, an additive fabrication method that utilizes electroplating of Cu on patterned photoresist mask is expected to be required for the precise control of fine patterns. For the fabrication of smaller devices or systems on flexible substrates in the near future, therefore, micro-scal
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