Face-up Interconnection Technique Using Direct Image Writing for Three-Dimensional Heterogeneous Flexible Electronics

We present direct image writing technique on flexible hybrid substrates for fabricating the face-up interconnection of three-dimensional heterogeneous flexible devices. In conventional metal wiring, the mechanical damage can be induced during the wire bonding process due to high contact pressure and ultrasonic energy for such thin film $(\le 10$ um) when processing on a flexible substrate such as a polymer. By utilizing an electrohydrodynamic jet printing system, it is possible to form a face-up interconnection without mechanical damage with silver nanoparticle based ink. We fabricated test-bed samples with step differences of 5, 7 and 10 um to determine the feasibility of face-up interconnection of 3D heterogeneous flexible devices. We confirmed that step height difference up to 10 um can be overcome by direct-imaging process. Resistance dependencies on the degree of bending with various bending condition on flexible substrate were investigated to evaluate the mechanical-electrical characteristics of face-up interconnection and confirmed that the printed silver metal wire can serve as robust interconnection up to 5 mm (bending diameter). For application demonstration, arrays of micro-LEDs (100 x 100 um) transferred onto a flexible polyimide substrate were interconnected by direct-imaging printing. Interconnected micro-LEDs showed typical diode I-V curves and had turn-on voltage of 3V. We confirmed that all devices were lit by applying a voltage higher than the turn-on voltage to the arrays of micro-LED. It was confirmed that the process was successful without damaging the flexible substrate in the electrohydrodynamic interconnection process. This work suggest the feasibility of using direct-image writing technique for face-up interconnection of three dimensional flexible electronics.