Integration of pentacene-based thin film transistors via photolithography for low and high voltage applications

Abstract

An organic thin film transistor (OTFT) technology platform has been developed for flexible integrated circuits applications. OTFT performance is tuned by engineering the dielectric constant of the gate insulator and the insulator/semiconductor interface. Full integration is enabled by a low temperature photolithographic patterning process that is compatible with flexible substrates. Devices and circuits for low voltage [ ... ] and high [ ... ] voltage applications are demonstrated. Both the low and high voltage OTFTs are made from the same set of materials and processes. Low voltage operation is achieved by the use of BZN (Bi1.5Zn1Nb1.5O7) which maintains a high dielectric constant (40) at low processing temperatures. With surface treatments and back channel encapsulation for patterning, OTFTs having two distinct threshold voltages (VT > 0 V and VT < 0 V) are integrated into logic inverters and ring oscillators based on logic inverters. To assess how BZN can serve as a gate dielectric in OTFTs, dielectric breakdown studies of BZN deposited at room temperature by RF Sputtering are presented. The time dependent dielectric breakdown (TDDB) and the time-zero dielectric breakdown (TZDB) are studied as a function of the polarity constant DC current stress, dielectric thickness, temperature, and surface treatments. Results show that current flows through these films via Schottky emission with a barrier height of ~1 eV on Au. Further, initial breakdown was not fatal and is characterized as a change in conduction mechanisms. This suggests that a trap assisted conduction mechanism dominates beyond a critical trap density (p = 1.5 x 1017 cm-3) which is generated due to electrical stressing. High voltage thin film transistor (HVTFT) switches very large drain-to-source voltages (VDD >300 V) with a lower controlling voltage (VG <20 V). An offset drain/source structure enables high voltage operation. A high voltage organic thin film transistor (HVOTFT) has been fabricated. As organic semiconductors and related devices are known for their compatibility with flexible media and/or large areas, the HVOTFT would be suitable for high voltage switching on such media. Gate insulator engineering is used to tune the threshold voltage and drain current in these devices. HVOTFTs of channel length 10 [mu]m and offset length 20 [mu]m suffer from non-saturating current behavior that is similar to the short channel effects reported in short channel OTFTs and Si-based MOSFETs, and a metastable charge injection similar to that reported in a-Si based HVTFTs.