Organic Materials for Next-Generation Devices

Researchers at Infineon Technologies (Munich, Germany) have analyzed various organic materials and processes that could be used to manufacture high-quality silicon-based memories as well as organic transistors and circuits. Use of conventional deposition processes and photolithographic patterning techniques will enable cost-effective manufacturing of these devices.

Infineon's researchers presented several papers on these fundamental technologies at the 2003 IEEE International Electron Devices Meeting (IEDM) in Washington, D.C. The encouraging results shown in the performance, reliability and temperature behavior of transistors, circuits and memories based on organic materials mark important milestones on the path to using organic materials for future electronic devices.

With regard to integration density and clock frequency, electronic devices using organic materials may not be a direct alternative to silicon-based integration, but they offer the potential of extremely low-cost manufacturing and a high degree of flexibility. While the production of ICs using silicon or other crystalline semiconductors requires weeks and a large number of sequential processes with expensive equipment, organic electronics can be manufactured at substantially lower cost. The potential uses of organic electronics are correspondingly low-performance and cost-sensitive applications.

Infineon demonstrated several organic components performing at levels previously unattainable. Thin-film transistors (TFTs) built by the researchers use organic semiconducting molecules as the active layer, and provide charge carrier mobilities in excess of 1 cm²/Vsec. Like silicon-based components, these organic transistors consist of several layers: substrate, gate electrode, gate insulator, source and drain contacts, organic semiconductors (e.g., pentacene or substituted oligothiophenes), and a protective passivation layer.

The researchers' report at IEDM included developments of both hybrid organic/inorganic structures and pure organic chips. "The latest research results at Infineon show that organic-based ICs have potential applications in high-volume and cost-critical applications where silicon-based chips may be unsuitable," said Christoph Kutter, senior vice president of Infineon's corporate research department. "The advances on performance and reliability demonstrated at the 2003 IEDM encourage us to continue our investigations of organic material for usage in new electronic devices."

Numerous organic and inorganic materials have already been analyzed for non-volatile memory applications. Organic memories offer the potential of simple integration and simple cell concepts with very small cell sizes. In contrast to inorganic materials, the properties of the organic memory layers can be tailored by a selective change of the molecular structure. Furthermore, organic materials are often suitable for vacuum deposition as well as for low-cost spin-coating processes.

Infineon researchers described the cell concepts and requirements for non-volatile memories based on novel organic memory materials (Figure ). Memory cells built with this technology have already shown promising reliability data. For the first time, retention data of more than a year are shown for an organic memory material exhibiting conductance switching. Further investigations show the potential for scaling the material down to feature sizes of <20 nm. This organic storage material is an attractive candidate for non-volatile memories.

Cross-point cell array with memory material deposited in vias. (Source: Infineon)

Traditional silicon integration concepts are based on a few materials like silicon, silicon oxide and silicon nitride. Using only these materials limits the integration options. Resists — also used in conventional semiconductor processes — could be a fourth alternative, but are limited because processing at higher temperatures is not possible.

Infineon co-developed a new thermally stable organic polymer combining the advantages of both worlds, and demonstrated the feasibility of this organic material for a DRAM trench integration scheme. In this approach, a modified version of an organic spin-on polymer is used with ideal gap fill properties, good planarization and temperature stability beyond 450°C. Test samples of 256 Mb DDR DRAM chips fabricated on 140 nm ground rules show high yields. This concept demonstrates the feasibility of front-end-of-line (FEOL) integration schemes using the newly developed material. Furthermore, the demonstrated integration scheme is capable of extending DRAM trenches to generations below 70 nm.

Infineon developed a new molecular TFT based on a high-mobility organic semiconductor (pentacene) and an ultrathin (2.5 nm), molecular self-assembling monolayer (SAM) gate dielectric.

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