High‐Mobility Organic Transistors with Wet‐Etch‐Patterned Top Electrodes: A Novel Patterning Method for Fine‐Pitch Integration of Organic Devices

Abstract

easily realized in the top-contact geometry, the limitation in minimum separation of source and drain electrodes prevents further advancement in on-off switching speed; it is essential to realize OFETs with a high-mobility semiconductor and a short channel length L , because the maximum circuit operating frequency is proportional to the transconductance g m and inversely proportional to L 2 . Moreover, the technique using shadow masks cannot be applied to mass-producible processes to fabricate integrated OFETs in an identical substrate with high yield.Photolithography is the well-established technique to fabricate fi ne patterns, though it is rarely employed in the top-contact OFETs because conventional organic semiconductor layers are seriously damaged by solutions used for common wet processes in the method. A method using orthogonal fl uorinated photoresist was proposed, [ 10,11 ] which enables to make fi ne patterns without dissolution of organic semiconductors. Though the method enables the fabrication of micro-and nano-scale OFETs, it has not matured yet to be applied for circuitries with short-channel and high-mobility semiconductor devices mainly because of the very large contact resistances. [ 11 ] In OFETs with bottom-contact geometry, photolithography is more often employed because the electrode patterning is fi nished before the semiconductor deposition. However, a major concern for the bottom-contact OFETs with a polycrystalline semiconductor fi lm relates to the molecular disorder occurring in the vicinity of the metal electrodes. [ 12 ] To overcome this problem, electrode surface modifi cations on gold fi lms by thiol-based selfassembled monolayers [ 13,14 ] or UV/O 3 treatment [ 15 ] are effective to reduce the contact resistance, however, these devices suffer from irreproducibility and poor long-term reliability due to chemical instability of most Au-thiolate SAMs. [ 13 ] In this communication, we report for the fi rst time a micropatterning process based on photolithography with simple wetetching of gold electrodes deposited directly on pristine organic fi lms without any adverse effect of residual photoresist. Since the wet-etching of gold is governed by iodide/iodine redox reaction, it is essential to use organic semiconductor materials that are robust to the reaction. Employing a newly developed highmobility organic semiconductor materials with tuned ionization potential, these processes are indeed suited to fabricate fi ne-pitch electrodes with low contact resistance, so that the highest cut-off frequency of 19 MHz is demonstrated for p-type transistors.In order to examine the robustness of organic semiconductors to the gold wet-etching process, we have carefully Based on the recent development of high-mobility and organic semiconductors, [1][2][3][4] organic fi eld-effect transistors (OFETs) offer promising prospects to realize such attractive applications as high-speed fl exible displays, fl exible radio frequency identifi cation (RF-ID) tags, and light-weight wearable smartsensing logics....

Injection Length in Staggered Organic Thin Film Transistors: Assessment and Implications for Device Downscaling

“…[2][3][4][5][6][7][8][9][10][11][12] Therefore by scaling the channel length down to 10-1 μm, in principle OTFTs should be able to operate at relatively high (1-10 MHz) frequencies at reasonable (<10 V) applied voltages. [ 13 ] In practice, apart from some reports, [13][14][15][16][17][18][19][20][21][22] this is often not easy to achieve because of injection issues: [ 23,24 ] contact resistances tend to become dominant over the channel resistance in short channel transistors reducing the expected improvement of device performances. [ 25 ] This situation is severely limiting the range of applications for OTFTs.…”

Accessing MHz Operation at 2 V with Field‐Effect Transistors Based on Printed Polymers on Plastic

“…Several research groups in fact have proven the feasibility of operating organic transistors at frequencies in excess of 10 MHz, [31][32][33][34][35][36] but only a few works have achieved such frequencies by adopting printing techniques, [37] which are more easily upscalable and compatible with large-area processing. Examples of f t beyond the MHz threshold have been shown through the adoption of conventional thermally grown silicon dioxide dielectrics (20 MHz at a bias voltage of 10 V [19] ), alumina deposited via atomic layer deposition (19 MHz at 10 V [33] ), or hybrid metal-oxide/self-assembled ultrathin dielectrics Organic printed electronics are suitable for the development of wearable, lightweight, distributed applications in combination with cost-effective production processes. [15,17,18] Moreover, since f t is proportional to the bias voltage, the requirement of low-voltage operation, at least below 10 V as necessary for portable, self-powered wireless electronics, further complicates the achievement of high operational frequency.…”

Direct-written polymer field-effect transistors operating at 20 MHz

“…However, while promising results in AC operation of organic FETs have been reported, with a maximum operating frequency as high as 27.7 MHz29, record values have been so far mainly achieved by adopting photolithographic steps30313233 and/or single crystal semiconductors34. To date, one order-of-magnitude lower operating frequencies have been demonstrated when mask-less, scalable techniques were used, with a maximum of 3.3 MHz25353637.…”