Infineon Advances Polymer Chips

Transistors made of organic materials such as polymers (i.e. plastics) will never compete directly with those made of silicon because they are relatively slow. But there are some very interesting applications where they could be essential, including signage, wireless tags (RFID tags), flat-panel displays, large sensor arrays and biochemical sensors. Some are even working on an "electronic newspaper" that will download the latest news stories and then be rolled up and put away (see Semiconductor International, November 2001, September 2001, August 2001 ). The main advantage of polymer transistors is that they conceivably can be fabricated very inexpensively, in a roll-to-roll process like potato chip bags.

Infineon Technologies  (Munich, Germany) recently announced that the company's research laboratories have developed and produced organic transistor-based electronic circuits on various substrates. These circuits have achieved performance capabilities that were previously unattainable. The thin-film transistors (TFTs) achieve charge carrier mobilities in excess of 1 cm²/Vsec. "The latest research results at Infineon show that plastic-based integrated circuits can be a potential addition to silicon chips in high-volume and extremely cost-critical applications," said CTO Soenke Mehrgardt. "Possible applications for low-cost polymer ICs are RFID tags as an alternative to bar codes or for use in the wearable electronics area."

Plastic thin-film transistors have a variety of uses, including wearable electronics, RFID tags, and electronic newspapers. (Source: Infineon Technologies)

The researchers' main goals were to substitute SiO₂ with a polymer gate insulator, and to manufacture transistors and circuits on a wide range of substrates, including low-cost, flexible plastic film. To achieve these goals, formulations and processes have been developed for the fabrication of high-mobility organic TFTs using polyvinyl phenol as the organic gate insulator and pentacene as the organic semiconductor. In this case, a charge carrier mobility of 3 cm²/Vsec was measured using thermally evaporated gold layers for the source and drain contacts; this is the highest mobility ever measured for organic TFTs. Using these pentacene TFTs, the researchers successfully manufactured digital circuits with a maximum clock frequency of 45 kHz (for circuits on flexible plastic substrates) and 65 kHz (for circuits on rigid glass substrates). Higher clock frequencies can be obtained by reducing the critical TFT dimensions — currently 5 µm — to 1 µm, but a minimum feature size below ~5 µm is unrealistic for low-cost, flexible substrate mass production.

Another goal accomplished by Infineon is the fabrication of transistors and circuits made entirely of organic materials, i.e. without the use of inorganic insulators and inorganic metals. This was achieved by using the synthetic metal PEDOT:PSS, and the charge carrier mobility of 0.3 cm²/Vsec measured for these devices is a record for TFTs with organic source and drain contacts.

The researchers say they have developed a broad portfolio of processes that can be combined in a variety of ways, and used to manufacture high-quality organic transistors and circuits using conventional deposition processes and photolithographic patterning techniques. They have also succeeded in using microcontact printing (a relief printing method similar to flexographic printing) to pattern transistors and circuits, with feature size and electrical performance similar to those of transistors and circuits fabricated using photolithography.

According to Infineon, there are several ways to market the new technology. The plastic chips will be glued to packaged goods in the form of finished labels, or the semiconductor industry will supply the packaging film ready with integrated electronic components for further processing. Last but not least, it will be the packaging industry's turn to bring the development to market.

For additional information on emerging technologies, go to www.semiconductor.net/emerging.