Development of scalable manufacturing processes for organic CMOS transistor circuits using self-aligned inkjet printing
Since interest in printed and organic electronics started to build two decades ago, one of the most compelling, albeit challenging, applications has proven to be printable organic transistors. The technology has significant application potential in several markets as it paves the way for simple intelligent devices that can be embedded into clothing, toys, games and everyday items. Smart labels, backplane circuits for next generation flexible displays and transducers for biological or chemical sensors are just some examples of how the technology could be used in the coming years. Once flexible displays, the first generation of polymer electronic products that are now close to industrial production, are validated successfully in the marketplace, the much anticipated market pull for organic transistors in other applications and sectors will intensify.
Printed polymer transistor technology currently still falls short of the performance requirements of many applications beyond a first generation of applications in flexible, reflective displays. A lot of research in recent years has focussed on improving basic transistor performance through use of novel materials and impressive demonstrations have been made of organic integrated circuits fabricated by conventional lithographic techniques, for example by the Holst Centre in the Netherlands. Professor Henning Sirringhaus, within the Cavendish Laboratory, observes: ‘When incorporating these materials into scalable printing processes, circuit performance is usually found to be much lower. This has prevented the use of organic transistors for applications in smart objects, and companies, such as Plastic Logic, have focused instead on display applications, for which adequate performance has been achievable.’
Sirringhaus’ team developed a scalable, high-performance printing-based manufacturing process for polymer transistor circuits, using an approach for downscaling printed TFTs called self-aligned printing (SAP).
The SAP technology, protected by several patent applications, can produce high-performance printed thin film transistors (TFTs) operating below 5V. With this technology the group was able to fabricate n-type and p-type self-aligned printed transistors with comparable mobilities, the building blocks for CMOS integrated circuits that are needed for more complex printed logic applications such as radio frequency identification (RFID). CIKC provided the funding that Sirringhaus and his team needed to take discoveries further and to develop logic capability that industrial partner Plastic Logic is using to develop interactive labels. Recently, the team identified several simple applications in wearable electronics and games and toys that required low cost interactive labels to achieve novel effects.
‘We’ve had more meaningful dialogue and provided Plastic Logic with more relevant technology,’ says Sirringhaus. This has led to a collaborative project, funded by the Technology Strategy Board (TSB), where smart label applications are being developed for commercialisation by CIKC and Plastic Logic with industrial partners. The label components – logic, display and sensors – are made using printable organic inks. The Functional Integrated Plastic Systems (FIPS) project is developing printable electronic labels for brand-enhancement applications, with a leading fast-moving consumer goods (FMCG) brand owner and the Centre for Process Innovation as well as other partners.
‘CIKC funding has meant that we could establish a team of two to three engineers over a five year duration to develop key manufacturing processes and a practical approach to system integration. This has allowed the group to work on a level that is internationally competitive,’ adds Sirringhaus.
Mike Banach, Research Manager at Plastic Logic, comments: ‘The circuit design and application-specific know-how of Henning’s team created a perfect synergy with our mature transistor technology. Together we can now engage with a new community of end-users for flexible electronics. The collaboration allowed us to tap into expertise that we didn’t have in-house and make progress very quickly.’