Organic field-effect transistors with nonlithographically defined submicrometer channel length

Abstract

We developed an underetching technique to define submicrometer channel length polymer field-effect transistors. Short-channel effects are avoided by using thin silicon dioxide as gate insulator. The transistors with 1 and 0.74 m channel length operate at a voltage as low as 5 V with a low inverse subthreshold slope of 0.4-0.5 V/dec, on-off ratio of 10 4 , and without short-channel effects. The poly͑3-alcylthiophene͒'s still suffer from a low mobility and hysteresis does occur, but it is negligible for the drain voltage variation. With our underetching technique also device structures with self-aligned buried gate and channel length below 0.4 m are fabricated on polymer substrates. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1758775͔Organic field-effect transistors ͑OFETs͒ are of interest for many applications including organic displays, complementary circuits, and all-polymer integrated circuits. 1-3Low-cost fabrication requires the use of solution-processible polymers instead of vacuum deposited low-molecular weight organics. Although mobilities of up to 0.1 cm 2 /V s have been demonstrated for soluble regioregular poly͑3-alkylthiophene͒, 3,4 poly-͑9,9-dioctylfluorene-bithiophene͒, 5 and pentacene from soluble precursors, 6 OFETs needed for high-performance circuits still require extremely small gate length to be modulated at competitive frequencies. The cutoff frequency f 0 ϭg m /(2C GS ) is determined by the maximum transconductance g m and the gate-source capacitance C GS . Neglecting parasitic capacitances, the latter is approximated by the gate oxide capacitance. Using the simple Shockley current characteristics one obtains as an estimation f 0 ϽU GS,eff /(2L 2 ), where U GS,eff is the gate-source voltage relative to the threshold voltage. With a voltage of at most 10 V, a realistic upper value of 0.01 cm 2 /V s for the mobility and a lower limit for the cut-off frequency of 100 kHz, one obtains as an upper limit for the channel length L Ͻ4 m. 10 used embossing to fabricate vertical-channel fieldeffect transistors with submicrometer channel lengths, but could not observe saturation in the measured output characteristics. These authors also presented a self-aligned gate structure with reduced overlap capacitance. We demonstrate the fabrication of submicrometer field-effect transistor channels by employing an underetching technique for submicrometer patterning, to enable the use of low-resolution photolithographic steps and standard microelectronic processes. Transistor design and dimensions are closely related to the material properties. At first, in order to reach the off-state and saturation, the active layer thickness must be less than the depletion length. 11 We use poly͑3-octylthiophene͒ ͑P3OT͒ and poly͑3-hexylthiophene͒ ͑P3HT͒, which are both unintentionally highly doped ͑of the order 10 17 cm Ϫ3 Refs. 11-13͒. To successfully deplete the layers their thickness must be of the order of 30 nm or less, which can be achieved by controlling the solution concentration and the spincoating process.11...

Current Trends in Shrinking the Channel Length of Organic Transistors Down to the Nanoscale

“…Rogers et al [72] integrated near-field photolithography with microcontact printing and shadow masking to demonstrate complementary inverter circuits from OFETs with 100 nm channel length. Scheinert et al [69] developed an underetching technique to define submicrometer channel length polymer FETs (Fig. 7B).…”

“…With great effort numerous approaches have been used to make nanoscale metal electrodes in order to achieve OFETs with submicrometer channel length. These methods include e-beam lithography, [55][56][57][60][61][62][63][64][65][66][67] shadow deposition, [68] underetching, [69] nanoimprinting, [70] rubber stamping, [71] and microcontact printing. [72] Device characteristics of OFETs made by each method are listed in Table 1.…”

“…The incorporation of molecular selfassembly into the device fabrication allows us to build effective field effect nano-transistors. These three systems are i) nanoscale metal electrodes (made using electron beam (e-beam) lithography, [55][56][57][60][61][62][63][64][65][66][67] shadow deposition, [68] underetching, [69] nanoimprinting, [70] rubber stamping, [71] and microcontact printing, [72] ), ii) single-walled carbon nanotube (SWCNT) electrodes, [5,54,73- and iii) cut single-layer graphene sheets as 2D contacts. [81] We will describe the fabrication of these devices as well as some of their unique characteristics and sensitivities.…”

σ-π molecular dielectric multilayers for low-voltage organic thin-film transistors

“…This C i effect on I DS is important for OTFTs considering the modest values of organic semiconductors, which are typically Ͻ1 cm 2 ⅐V Ϫ1 ⅐s Ϫ1 vs. 10 3 cm 2 ⅐V Ϫ1 ⅐s Ϫ1 for crystalline Si. Furthermore, high-quality ultrathin gate dielectrics will be essential in combination with small channel lengths for high frequency operation (11,12) and self-assembled molecular electronics (refs. 13 and 14 and references therein).…”

Vertical polyelectrolyte-gated organic field-effect transistors

“…Many fabrication techniques have been explored to achieve organic transistors with short channels, for example, self-aligned inkjet printing, 1 selective laser sintering, 2 hot embossing, 3 UV-nanoimprinting, 4 electron-beam lithography, 5 and underetching. 6 However, the processing complexity of high-resolution manufacturing equipment typically prevents large-area and high-volume production. One possible option to achieve small channel dimensions is to stack metal/insulator/metal layers to define the source-channel-drain configuration in a vertical manner.…”