Contact effects in thermally evaporated pentacene thin films and aspects of microsystem hybrid integration

Organic thin film transistors have the potential to replace silicon based transistors in applications such as smart cards and RF-ID due to their low cost and low processing temperatures. Thermally evaporated pentacene is studied as an organic thin film material. Thin film transistors were fabricated in bottom contact structure using thermal evaporation of pentacene at a rate of 0.5 to 1 nm/s. Heavily doped Si was used as a gate material and 100 nm thick silicon dioxide was used as a dielectric. Ni was used as contact metal for source and drain contacts. Threshold voltage of 16 V and mobility of 0.0016 cm2/V-s were obtained. Grain size in pentacene films increased from 120 nm to 150 nm upon annealing at 200 ◦C for 30 min. in nitrogen ambient. Resistivity of the pentacene films decreased with annealing temperature indicating an activation energy of 0.22 eV. Hybrid Integration of a Bio-implantable Electrical Stimulation System (BESS) is carried out in the second part of this work. BESS produces periodic pulses that stimulate the gastric muscles. BESS consists of an application specific integrated circuit powered by rechargeable batteries, which are charged by a remote power delivery system. Screen printing technique was used for BESS hybrid integration. The same can be extended in future for fabricating organic transistors. The optimum screen printing process determined included squeegee speed of 2.2 cm/s, off-contact height of 2 mm and squeegee pressure of 80 PSI for ED3000 ink used. Ceramic, silicon, glass, polyimide and kapton substrates have been utilized for printing. Interconnect pattern of area 1” × 0.75” was screen printed on a ceramic substrate using ED3000 silver conductive ink and surface mount components were mounted. Bio-compatible 100 µm thick polyimide substrates were prepared by spin coating pyralin at 750 RPM and baked at 350 ◦C for 30 min. ED3000 conductive ink was used to print electrodes on polyimide substrates. The BESS system is now ready for full hybrid integration and testing