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Researchers at the University of Cambridge's Cavendish Laboratory have now used a clever trick to achieve that higher resolution, allowing them to print all-polymer transistor circuits [Science, 290, 2123 (2000)]. The technique appears to bring the promise of low-cost, printable organic electronics one step closer to realization, according to another electronics researcher, Dagobert M. de Leeuw of Philips Research Laboratories in Eindhoven, the Netherlands. To print an organic transistor circuit, an ink-jet printer is used to spray droplets of the ink--a water-based conducting polymer--from a nozzle onto a hydrophilic substrate such as glass. The extent of the spray is somewhat variable, however, and the droplets can spread on the substrate. These factors limit how close together two electrode lines can be deposited without producing a short circuit between them, explains physicist Henning Sirringhaus , a lecturer at Cavendish Laboratory. To overcome this problem, Sirringhaus, physics professor Richard H. Friend , and graduate student Takeo Kawase confine the spreading of the droplets on the hydrophilic substrate by using a pattern of narrow, repelling, hydrophobic surface features that define the critical dimensions of the device. These hydrophobic features are fabricated by patterning a polyimide film by conventional photolithography and oxygen plasma etching. In the organic thin-film transistor (TFT) made in this study, the critical feature is a pair of conducting polymer electrodes--known as the source and drain--that are separated by a 5-m-wide channel. To achieve this separation, the team simply patterns a polyimide line 5 m wide and then sprays a line of droplets of the aqueous conducting polymer--poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS)--on either side of this hydrophobic barrier. Atomic force microscopy images show that the ink-jet droplets coalesce into linear polymer electrodes that abut the polyimide barrier. In further fabrication steps, the researchers use spin-coating to cover the source and drain electrodes and the polyimide features with an active semiconducting polymer, poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). This polymer serves as a conduit for charge transport from the source to the drain. A film of a dielectric (insulating) polymer, polyvinylphenol, is then spin-coated on top. Finally, the ink-jet printer is used to deposit a gate electrode line of PEDOT/PSS directly over the source-drain channel. In an interview with C&EN, Sirringhaus stresses that the initial photolithographic patterning of the substrate with polyimide barriers isn't essential for confining the ink in the printing step. At the time this work was done, he says, "it was just a convenient way" to define contrasting areas on the substrate where the ink either could or could not spread. The same type of surface-energy contrast can be created, for example, by depositing a self-assembled monolayer in a particular pattern. This approach would allow the entire circuit to be fabricated by a sequence of solution coating and printing steps, he notes. The use of polyimide is advantageous, though, because in addition to precisely defining the narrow channel, it also serves a second function, adopted from liquid-crystal technology: When rubbed, the polyimide serves to align the molecules of the semiconducting polymer F8T2, leading to faster transport of holes (positive charges) through the semiconducting layer--that is, higher field-effect mobilities. In the printed device reported by researchers at Cavendish Laboratory, the source, drain, and gate electrodes are made of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS). A polyimide feature that is patterned on the substrate before the source and drain are deposited by ink-jet printing serves to define the channel, which is the separation between the source and drain. The semiconducting polymer is poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). The dielectric polymer is polyvinylphenol. [Adapted from Science] The Cavendish group reports achieving mobilities of 0.01 to 0.02 cm2 per volt-second--a "respectable" mobility for a polymer transistor, Sirringhaus comments. That figure is at least an order of magnitude lower than the mobility of conventional amorphous silicon TFTs, but it is similar to mobilities of devices containing photolithographically patterned gold electrodes. Achieving this mobility, the researchers point out in their paper, shows that "through careful choice of the sequence of solvents and polymers to avoid dissolution and swelling of underlying layers, our printing process maintains the critical integrity and abruptness required of the different polymer-polymer interfaces in a multilayer TFT device." When these printed polymer transistors are switched from the "on" to the "off" state, the current passing through the device diminishes by a factor of 105, which is "a reasonable value for a thin-film transistor," according to Sirringhaus. He expects that the performance of these transistors will improve as the research continues. Furthermore, he thinks that the printing process could be extended to form even finer features, possibly smaller than 1 m. This would lead to smaller and faster transistors. To fabricate complex electronic circuits using ink-jet printing, scientists will need to fashion not only thin-film transistors but other circuit components as well. Of particular importance are so-called via-hole or vertical interconnects, which, for example, connect electrodes in different layers. The Cambridge scientists have developed a process for fabricating via-hole interconnects using ink-jet-deposited solvents. A droplet of the solvent is first deposited on a polymer layer, causing the polymer at that site to dissolve, forming a tiny crater. As the solvent evaporates, the polymer is redeposited on the edges of the crater. This process is repeated several times until the surface of the underlying layer, which is impervious to the solvent, is exposed. The resulting via-hole is then filled with a semiconducting polymer using ink-jet printing. The ability to make via-hole interconnects in this new way has allowed the Cambridge team to fabricate inverters, which are the basic building blocks of a logic circuit. An inverter basically converts a high-voltage input to a low-voltage output, or the inverse. According to Sirringhaus, the inverter switching speed and the performance of the printed TFT circuits are comparable with that of all-polymer TFT circuits that have been reported by de Leeuw's group at Philips. De Leeuw and coworkers use a three-level photolithographic process to pattern electrodes and via-holes. Sirringhaus and coworkers believe that their ink-jet printing process can produce circuits of similar complexity. Moreover, it may lend itself to the continuous reel-to-reel processing of large-area circuits on plastic substrates. Such a reel-to-reel process, Sirringhaus explains, is like printing a newspaper: A continuous roll of substrate would pass through a succession of stations where solution coating, printing, and other steps are performed. By contrast, he notes, the standard way of making integrated circuits today is a batch process in which individual substrates are moved from station to station, where photolithography, etching, deposition, and other procedures are performed, some in a vacuum. The Cavendish researchers have not explored reel-to-reel processing because they aren't in an industrial lab and thus don't have the necessary equipment. But in principle, Sirringhaus tells C&EN, the group's ability to carry out all the fabrication steps using printing and solution-based processes makes reel-to-reel processing "an attractive proposition." |
| UPDATE | 01.01 |
| AUTHOR |
Uni. of Cambridge's Cavendish Lab. - Friend Richard H.; Physics Prof. - Sirringhaus Henning; Physicist & Lecturer |
| LITERATURE REF. | Science, p. 290, p. 2123 (2000) |
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