Fabrication and testing of polymer thin film transistors for basic digital circuits

Abstract

In this thesis we developed fabrication and characterization methods for polymer thin lm transistors(PTFTs) and circuits. After analysis of the various aspects related to the PTFTs, we propose a new method to fabricate self-aligned transistors on a transparent exible substrate. This method relies on the back-side exposure and allows self-aligned short-channel transistors to be fabricated in close proximity to not-self aligned ones. Self-aligned transistors with a length of 50 m and width of 1000 m have a saturation current of around 1 A and a saturation voltage of around -38 volts at a VGS of -40 volts. Following the self-aligned fabrication method, we use a regular process ow to fabricate fully patterned bottom-gate bottom-contact transistors on glass substrate. PTFTs with interdigitated, corbino and standard layout are fabricated. Gate dielectric and organic semiconductor are SU-8 and poly(3-hexylthiophene)(P3HT), respectively. Two methods for the patterning of the polymeric active layer are proposed. MIMIC is used to obtain P3HT patterns with several micrometer thickness. Lift-o , however, is found to be suitable for the patterning of relatively thin(20 to 100 nm) P3HT lms. Moreover, leakage current of about one twentieth of the channel current, resulting from the non-patterned active layer is eliminated successfully by P3HT patterning with both MIMIC and lift-o . A simple method for fabricating discrete transistors with silver epoxy drain/source electrodes is proposed next. PTFTs fabricated with this method showed a saturation mobility of 0:036 cm2=V s and ION=IOFF ratio of about 8. Finally, we examine a treatment route for de-doping of the P3HT lm. Vacuum treatment is found to be an e ective way for desorption of the doping species from P3HT, heat treatment after vacuum, however, did not result in the reduction of the lm conductivity. Electrical conductivity of drop-casted P3HT lm decreased from 2.2x10-5 Scm-1 to 0.5x10-6 Scm-1 after vacuum treatment. In uence of heat and light at di erent wavelengths on the conductivity of the lm is investigated at the last part of the thesis. Conductivity increased exponentially to 8x10-5 Scm-1 from initial 0.5x10-6 Scm-1 when temperature swept from 30 C to 140 C. Light with di erent wavelengths has also a observable e ect on lm conductivity. Exposure to blue light caused 9 fold increase in conductivity. Elimination of the light induced e ect took around 5 hr for each wavelength.