Project conclusion

Organic electronics have become a promising technology for applications requiring large-area coverage, mechanical flexibility and low temperature processing. The implementation of organic electronics in various market fields is in fast progress and the organic contribution to all-day electronic devices is forecasted to massively enhance within the next 20 years. The envisaged application fields are logic and memory, lighting, power devices, sensors, displays for electronic products and active and passive matrix components. 

As the plastic electronics field matures towards more advanced products such as smart objects and intelligent sensor networks, the obvious demands concern higher operation frequencies and the extension towards new substrate types as for example textiles and stretchable plastic foils. With commonly used low-cost industrial patterning and printing technologies, the maximum operation frequencies that can be expected for organic logic gates are in the 10 kHz range due to the limited resolution of these techniques; the minimum achievable channel length is in the range of 5 µm. A reduction of the critical dimensions of organic thin film transistors towards the submicron channel length regime opens new perspectives for switching frequencies in the MHz region. Apart from downscaling, the minimization of the parasitic capacitance contribution is another important prerequisite for the reduction of switching times. 

The overall objective of the project NILecho was the development of a technology for organic electronic complementary circuits on flexible substrates operating at frequencies far above 10 kHz. The dramatic increase of the switching speed of representative organic circuits is achieved by reducing the critical device dimensions and the parasitic contributions whilst strictly controlling the mobility. The technology is based on NIL hot embossing, which is an innovative and seminal reel-to-reel compatible, fast and parallel high-resolution patterning technique in the field of nanoimprint lithography. Parasitic capacitance contributions are minimized via self-alignment of source and drain electrodes with respect to the gate structures. A combination of short-channel, fast organic complementary circuits with a self-aligned nanoimprinting process has to be classified as radically innovative in the field of complex electronic applications opening a series of new potentials and fields of applications.

The use of a complementary technology comparable to the CMOS approach of the standard semiconductor industry leads to lower power dissipation, higher noise margin, better robustness and easier design of the circuits, but involves challenges as for example optimum crystalline growth of two organic semiconductors on the same substrate and the output current matching of p- and n-type devices. 
Parasitic capacitances in organic thin film transistors are predominantly formed by a non-vanishing overlap of electrodes and conduction lines that are outside the active transistor region but only separated by the gate dielectric layer. In order to minimize this contribution the overlap between the gate electrode with the source and drain electrodes should be as small as possible.
Nanoimprint lithography (NIL) is the only structuring technique combining high-resolution in the nanometer range, reel-to-reel compatibility and a parallel low-cost production. In the present case, hot embossing is used for structuring in these critical dimension levels with embossing temperatures below 150°C. The NIL process creates patterns by mechanical deformation of an imprint resist, which in the case of hot embossing typically is a polymer formulation with low glass-transition temperature. As the development of such resists is quite sophisticated by now, maximum process temperatures lower than 150°C are state-of-the-art.
As p-type semiconductor for the organic complementary circuits (inverters, ring oscillators), the organic small molecule pentacene has been chosen. Regarding its high charge carrier mobility and stability against air and humidity in comparison to other organic semiconductors, it is a leading candidate in the field of p-type organic semiconductors. It can be implemented in the NIL process for the fabrication of OTFTs in combination with organic as well as inorganic dielectrics and shows polycrystalline growth with required low nucleation density on the respective surfaces. As n-type semiconductor fluorinated copper-phthalocyanine is used, due to its high charge carrier mobilities up to 0.03 cm²/Vs. 
In order to minimize the overlap capacitances, a so-called self-aligned nanoimprinting process was developed, which involves an intrinsic alignment of the different electrode levels that reduces the lateral overlap of electrodes and conduction lines as well as supersedes the alignment of the stamp with respect to the gate structures.
Within the project all work packages have been addressed and all milestones have been successfully reached. The NILecho work started with the fabrication of organic thin film transistors - p-type and n-type - with channel lengths in the submicron range by use of NIL hot embossing. In parallel, a self-aligned process chain based on NIL, where the gate level automatically defines the source/drain-level of the transistor, was developed and defined. After successful device adjustment and drain current matching, the integration of these p- and n-type single devices to inverters as smallest building block for digital circuits was successfully realized and transferred onto flexible substrates. This work was accompanied by a detailed sample analysis by means of Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDX), Electron Energy Loss Spectroscopy (EELS) and Energy Filtered TEM (EFTEM) as well as a comprehensive device and circuit design and test equipment development.

The technical targets of NILecho combine a new technology for the fabrication of high-resolution organic electronic complementary circuits with high operating frequencies far above 10 kHz. Major attention was paid to the development of the nanoimprinted flexible organic inverter and the implementation of these inverters to ring oscillators. 
The fabrication of the worldwide first organic flexible complementary inverter with submicron transistor channels (Rothländer et al, JMR, submitted) and n-type nanoimprinted transistors fabricated by a self-aligned NIL approach with switching frequencies in the range of 400 kHz (Palfinger et al, Adv. Mat. 22, 5119-5119 (2010)) are two of the outstanding experimental successes of the NILecho project.