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Analog and Digital Circuits Using Organic Thin-Film Transistors on Polyester Substrates M. G. Kane, J. Campi, M. S. Hammond, F. P. Cuomo, B. Greening, C. D. Sheraw, J. A. Nichols, D. J. Gundlach, J. R. Huang, C. C. Kuo, L. Jia, H. Klauk, and T. N. Jackson
Abstract—We have fabricated and characterized analog and digital circuits using organic thin-film transistors on polyester film substrates. These are the first reported dynamic results for organic circuits fabricated on polyester substrates. The high-performance pentacene transistors yield circuits with the highest reported clock frequencies for organic circuits. Index Terms—Organic electronics, organic semiconductors, plastic substrates, thin-film circuits, thin-film transistors.
I. INTRODUCTION
O
RGANIC thin-film transistors (OTFTs) are gaining attention as a technology that enables electronic circuits and displays to be fabricated using low-cost processing on plastic substrates. The entire fabrication process can be performed at temperatures of about 100 C or less, allowing great freedom in the choice of substrate materials. Plastic substrates, in particular, allow circuits to be fabricated directly onto smart cards and inventory tags, and open up the possibility of rapid, highvolume web processing. Rugged, flexible displays can also be built using an OTFT active-matrix on plastic, with liquid crystal or organic light-emitting diodes (OLEDs) as the electro-optic element. OTFT circuits have been reported on silicon substrates [1], [2], on glass [3], [4], and on polyimide, a high-temperature engineering polymer [5]. Functional circuits containing as many as 864 transistors have been built [6]. Single “smart pixels” have also been fabricated by combining an OTFT with an OLED [7], [8]. The first organic circuits on low-cost polyester film were recently reported, consisting of single two-transistor inverters and smart pixels [9]. Here we report analog and digital organic circuits with complexities up to 48 transistors fabricated using pentacene OTFTs on polyester film. We present the first reported dynamic results for organic circuits fabricated on polyester film. The high performance of our organic transistors yields circuits with the shortest gate-delay reported for organic circuits on plastic substrates, and the highest clock frequencies reported to date for any organic circuits. Manuscript received May 1, 2000. This work was supported by the Defense Advanced Research Projects Agency under Contract N61331-98-C-0021. The review of this letter was arranged by Editor T.-J. King. M. G. Kane, J. Campi, M. S. Hammond, F. P. Cuomo, and B. Greening are with the Solid State Display Laboratory, Sarnoff Corporation, Princeton, NJ 08543 USA (e-mail:
[email protected]). C. D. Sheraw, J. A. Nichols, D. J. Gundlach, J. R. Huang, C. C. Kuo, L. Jia, H. Klauk, and T. N. Jackson are with the Electronic Materials and Processing Research Laboratory, Pennsylvania State University, University Park, PA 16802 USA. Publisher Item Identifier S 0741-3106(00)09276-4.
II. ORGANIC TFT STRUCTURE AND FABRICATION PROCESS The substrate material was a 75 m-thick transparent, flexible polyethylene naphthalate (PEN) film mounted on a removable glass support for ease of processing. After an initial 150 C, 2 h heat treatment to improve thermal dimensional stability, the maximum processing temperature was 110 C (photoresist bake). Nickel gate metallization, a 190 nm SiO gate dielectric, and palladium source-drain metallization were deposited and patterned. The surface of the gate dielectric was vapor-treated with octyldecyltrichlorosilane, followed by thermal evaporation of the active pentacene layer at a substrate temperature of about 60 C. Further details of the pentacene OTFT process can be found in [10]. To pattern and passivate the pentacene layer without exposure to common solvents and resist developers, a water-based solution of polyvinyl alcohol (PVA) with ammonium dichromate photosensitizer was spun onto the substrate, UV-exposed through a chrome mask, and developed in water. Pentacene outside the active TFT regions, unprotected by the patterned PVA, was removed in an oxygen plasma. Because our OTFTs are typically depletion-mode, unless the pentacene islands are isolated there is excessive off-state leakage through the field regions; the only alternative is to use a geometry in which the source surrounds the drain, but this requires a third level of metallization in order to contact the drain. The PVA process isolates the OTFTs effectively without significantly modifying device characteristics. The passivation layer also permits different types of organic devices to be integrated on one substrate, since it can protect lower organic layers from the deposition and patterning of upper layers. In addition, passivation will be important for liquid-crystal display applications, because we have found that pentacene OTFTs are degraded when they come into contact with typical liquid-crystal materials. Each substrate includes a 1 cm array of 200 transistors with 25 m and 20 m. Typical drain-current characteristics of a passivated device are shown in Fig. 1. A high degree of device uniformity is possible. On one array, the average threshold voltage was +3.2 V and the average field-effect mobility was 0.45 cm /V-s, with a standard deviation of 0.2 V for threshold voltage and 0.03 cm /V-s for mobility. III. ORGANIC CIRCUITS Several types of analog and digital circuits were fabricated and characterized. All the circuits used transistors with 10 m. Yields were acceptable, with 65% yield, for example, on 48-transistor divide-by-two frequency dividers. Because the
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KANE et al.: ORGANIC THIN-FILM TRANSISTORS ON POLYESTER SUBSTRATES
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Fig. 1. Drain-current characteristics of a typical passivated pentacene OTFT on a flexible polyethylene naphthalate (PEN) substrate. The device is a depletion-mode p-channel transistor, with a threshold voltage of +6 V and a field-effect mobility of 0.6 cm /Vs. Fig. 3. Transfer characteristics of an organic differential amplfiier. The gate of the tail current source was brought out to an external pad for adjusting the bias is plotted current. The voltage of one of the differentially driven inputs V on the horizontal axis, and the voltage of the two outputs V and V are plotted on the vertical axes. The differential voltage gain in the region of maximum gain is 8.5. The inset shows the schematic of the amplifier. Supply V and V V V. voltages are V
0 = 020
Fig. 2. Logic diagram and operation of the organic divide-by-two frequency divider. The top trace shows one of the two nonoverlapping complementary clock input signals, with a frequency of 1.1 kHz. The bottom trace shows the two complementary output signals. (Vertical scales are 2 V/div.).
transistors were depletion-mode, in addition to an input stage each logic gate included a two-transistor level-shifting output stage using a source-follower biased by a current source. We measured a maximum oscillation frequency of 1.7 kHz on fivestage ring oscillators operated at 20 V, corresponding to a 59 s gate delay. This is the shortest gate-delay reported for organic circuits on plastic substrates. Fig. 2 shows the logic diagram and operation of the divide-by-two frequency divider. This circuit provides a more realistic assessment of speed than ring oscillators, since proper functionality requires large internal voltage swings that approach saturated binary logic levels. At a supply voltage of 25 V the circuit operated at 1.1 kHz, the highest operating frequency reported to date for clocked organic circuits. Fig. 3 shows results from an organic differential amplifier consisting of a source-coupled pair with depletion loads, biased by a current source. Transfer characteristics are shown for
=
= +10
both outputs with differential input drive. Both sweep directions are shown in order to note that the amplifier typically exhibits some hysteresis, possibly due to slow trapping states at the pentacene-SiO interface. By averaging the characteristics for the two sweep directions, the input offset voltage is found to be 0.6 V. In the high-gain region the differential-mode gain is 8.5. On other amplifiers the input offset ranged from 1 to 1 V, and the differential gain from 5 to 10. The low voltage gain of our OTFT amplifiers results from to drain conductance a small ratio of transconductance in the saturation region (Fig. 1). The differential gain could be increased by adding cascode transistors to raise the output impedances of the gain stage and the current source, at the expense of supply voltage. Low gain also affects the logic gates. Our inverters typically have an input-stage voltage gain of about 3 in the region of highest gain. The unity-gain level-shifters have a gain of about 0.75, leading to an overall inverter gain of only about 2.25. The low gain reduces noise margins, especially in gates with multiple inputs. In logic gates cascoding is impractical because it significantly increases transistor count and power dissipation, and a better solution may be to use complementary n- and p-channel transistors [11]. Alternatively, new materials and processes may be required. IV. CONCLUSION Our results demonstrate the usefulness of OTFT technology for fabricating analog and digital circuits on low-cost polyester substrates. Ring oscillators yield gate delays under 60 s, the shortest gate-delay reported for organic circuits on plastic substrates. Clocked circuits function at frequencies above 1 kHz, the highest operating frequency reported to date for clocked organic circuits. Future work is needed to increase the ratio of
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transconductance to drain conductance in order to improve the voltage gain of analog circuits, and the noise margin of digital logic.
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[4] H. Klauk, D. J. Gundlach, and T. N. Jackson, “Fast organic thin-film transistor circuits,” IEEE Electron Device Lett., vol. 20, pp. 289–291, 1999. [5] C. J. Drury et al., “Low-cost all-polymer integrated circuits,” Appl. Phys. Lett., vol. 73, pp. 108–110, 1998. [6] B. Crone et al., “Large-scale complementary integrated circuits based on organic transistors,” Nature, vol. 403, pp. 521–523, 2000. [7] H. Sirringhaus, N. Tessler, and R. H. Friend, “Integrated optoelectronic devices based on conjugated polymers,” Science, vol. 280, pp. 1741–1744, 1998. [8] A. Dodabalapur et al., “Organic smart pixels,” Appl. Phys. Lett., vol. 73, pp. 142–144, 1998. [9] J. A. Rogers, Z. Bao, A. Dodabalapur, and A. Makhija, “Organic smart pixels and complementary inverter circuits formed on plastic substrates by casting and rubber stamping,” IEEE Electron Device Lett., vol. 21, pp. 100–103, 2000. [10] H. Klauk, D. J. Gundlach, J. A. Nichols, and T. N. Jackson, “Pentacene organic thin-film transistors for circuit and display applications,” IEEE Trans. Electron Devices, vol. 46, pp. 1258–1263, 1999. [11] Y.-Y. Lin et al., “Organic complementary ring oscillators,” Appl. Phys. Lett., vol. 74, pp. 2714–2716, 1999.
