Technology Development for Direct-Printed Dual-Gate Organic TFT Circuit Fabrication with 3-D Integration

The direct printing of organic thin-film transistors (TFTs) gives us a facile and fast route to fabrication of freeform electronic applications on a flexible substrate. However, despite the significant improvement of carrier mobilities of organic semiconductors, electronics industry still has not adopted the printed organic TFTs for manufacturing electronic applications. The low resolution, large feature size, and variation of inkjet printing process have limited the implementation of organic integrated circuits with reasonable transistor densities and uniformity. To overcome these problems, we have developed robust inkjet design rules for high-yield circuit pattern generation and a three-dimensional (3-D) integration process for high-density printed dual-gate (DG) organic TFT circuit fabrication-complementary TFTs are stacked in transistor-on-transistor manner. In this work, all the functional layers including electrodes and semiconductors are fully printed except a Parylene dielectric that is formed by chemical vapor deposition. The design rules have been developed via a systematic study on the influence of inkjet printing parameters (substrate temperature and drop spacings) on the minimum width, smoothness, and reproducibility of printed metal lines in vertical and horizontal axes. The stacked DG TFTs showed average carrier mobilities of around 0.1 and 0.7 cm&2hat;/(Vs) for the n-type and p-type TFTs, respectively, showing exceptional uniformity and long-term stability at 5 V operation voltage. We successfully fabricated 7-stage ring-oscillator on a flexible PEN film with the maximum frequency of 200 Hz and a gate delay of 340 us and also demonstrated 3-D NAND-based logic design. We believe our printed circuit design approach based on the robust printing rules provides a path to systematic printed flexible circuit design with high transistor density, high yield, and high uniformity which are critical for the realization of printed organic electronics applications.