Solution-based transistor matrix

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Susan Banks
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1 18 PRINTED ELECTRONICS Solution-based transistor matrix A groundbreaking new technology is making it far more cost-effective to produce the electronic control unit of liquid crystal displays. At the same time, Evonik is showing display manufacturers a development path to future production processes for printed electronics. The development started in the S2B Center Nanotronics of Creavis and about 20 people were transferred in 2012 to Evonik s Coatings & Additives Business Unit. Furthermore, it also marks the first time since the establishment of Creavis, whose mission is to create new business activities, that a project of this scope has been transferred to a business unit. [TEXT: Dr. Ralf Anselmann, Dr. Jürgen Steiger] A revolution has taken place in the world of flat screens in the past twenty years. After decades in which the dimension and picture quality of televisions and computer monitors were determined by cathode ray tubes, the overall depth of the displays has now shrunk even more, while the screen size has continuously increased. And thanks to smartphones, small, high-resolution displays have also gained massive importance in recent years and are now by far the largest segment of the display market in terms of units sold. The end of these technol ogical developments is nowhere in sight: Industry and consumers are demanding more innovations. Displays are produced by various processes based on the application and the admissible costs and, obviously, the technical feasibility. According to the market research firm Display Search, liquid crystal displays (LCDs) have by far the largest market share, accounting for about 80 percent of the market. Typical fields of application include televisions, notebooks, and smartphones. There are also other display technologies with a notable market share, including electrophoretic display types such as those used in most e-readers, or organic light-emitting diodes (OLEDs), which are used for such applications as smartphone displays. The one thing they all have in common is that the pixels normally can be individually controlled, owing to a matrix consisting of thin-film transistors (TFT) called a TFT backplane. Because each pixel is made of three subpixels in the basic colors of red, green, and blue, for the sake of displaying color, three transistors are required to control a single pixel elec tronically. And because HDTV-compatible televi sions, which are now the standard for new devices, have resolutions of 1,920 x 1,080 over two million pixels their backplanes contain about six million TFTs. Today, display sizes of more than 40 inches are common, with 32 inches being the standard. Display manufacturers have to be able to make TFT backplanes large enough to meet cost and quality objectives. The industry is already considering screens with four times the resolution of full HDTV or even more. One driver for such developments is the 3D display, which often requires at least double the number of pixels to achieve HDTV resolutions. After all, they have to offer a high-resolution view to each eye. The backplanes must then accommodate proportionately more thin-film transistors to control the pixels with the same technology over 24 million when it comes to Quad HD resolution, for 333

Evonik works in clean room facilities in Marl on new oxidic semiconductors for thin-film transistors that require no vacuum technology.

Thin-film transistors produced in this way will not only make flat screens easier and less expensive to produce, but pave the way for the

Advanced HDTV televisions have a resolution of 1,920 x 1,080 pixels, each of which are controlled by three TFTs in the basic colors of red,

2 Evonik works in clean room facilities in Marl on new oxidic semiconductors for thin-film transistors that require no vacuum technology. Instead the materials can simply be applied from solution to the substrate. Thin-film transistors produced in this way will not only make flat screens easier and less expensive to produce, but pave the way for the high-resolution televisions of tomorrow Simplified structure of an LCD screen. Advanced HDTV televisions have a resolution of 1,920 x 1,080 pixels, each of which are controlled by three TFTs in the basic colors of red, green, and blue for the purpose of displaying color. Such devices therefore contain 6,220,800 thin-film transistors Polarizer Glass plate RGB color filter Electrodes Orientation layer Spacer Liquid crystal Orientation layer Thin-film transistor (TFT) Electrodes Glass plate Polarizer Light source

3 20 PRINTED ELECTRONICS An Evonik employee collecting measurement data. In the clean rooms, materials for thin-film transistors are not only prepared under clean room conditions, but they are also measured electron ically. Assessing the results of the electronic measurements directly after production is an important factor for ensuring fast feedback to the mate - r ial devel opers 3D displays mean increasing demands on resolution example. And since the refresh rates are also increasing, TFTs will have to switch significantly faster. It is doubtful whether this is possible with the transistor material most often used nowadays, amorphous silicon. A measure for the switching capability of a thinfilm transistor is the charge carrier mobility of the semiconductor. For electrophoretic displays, such as those in ebook readers like Amazon s Kindle, a charge carrier mobility of significantly less than one square centimeter per volt and second are sufficient: E-readers do not have to be able to play back moving pictures or change pictures rapidly. The charge carrier mobility for an LCD TV, on the other hand, should be between 1 and 10 square meters per volt and second depending on the quality standard. Values higher than 1 square meter per volt and second cannot be achieved with amorphous silicon or only at prohibitively high costs. Displays made of organic light-emitting diodes, the kind built into various smartphones, even require semiconductors with a charge carrier mobility of more than ten square centimeters per volt and second. For these, manufacturers are now primarily using transistors made of polycrystalline silicon, because of its much higher charge carrier mobility. But crystalline materials naturally produce inhomogenities that limit the size of the backplanes that can be produced. This is because the mobility of the charge carrier is reduced at the boundaries that are generated between different crystal orientations in the layer. The advantage of amorphous silicon is precisely that, because of its structure, it can be applied very homogeneously to large surfaces. Even though there is silicon available today low-temperature polycrystalline silicon that stands out for its extremely high charge carrier mobility, it is still extremely expensive to use in the production of TFT backplanes. An alternative to amorphous silicon that promises significantly higher charge carrier mobility are amorphous metal-oxide semiconductors such as indium-gallium-zinc-oxide (IGZO). The charge carrier mobility of this material class is in the area of ten square centimeters per volt and second enough to meet future switching requirements in LCD and OLED screens. In recent years, various manufacturers have already presented prototypes at industry conferences. These semiconductor materials are considered highly promising, which is why display manufacturers are currently qualifying and selecting production units and processes. These kinds of metal oxides and amorphous silicon can be applied in such processes as 333

PRINTED ELECTRONICS 21 EU PROJECT ORICLA A benchmark for printed RFID logistics As part of a research project, the printed electronics working group in the Science-to-Business Center Nanotronics also

in the technology currently found in all conventional computer chips.

4 PRINTED ELECTRONICS 21 EU PROJECT ORICLA A benchmark for printed RFID logistics As part of a research project, the printed electronics working group in the Science-to-Business Center Nanotronics also worked with partners to develop materials for a RFID technology to serve as a benchmark in the industry: A bidirectional circuit based on organic and metal-oxide semiconductors in complementary logic in the technology currently found in all conventional computer chips. The EU-funded project is called ORICLA, and its partners include the Belgian Research Institute IMEC, the Netherlands Organization for Applied Scientific Research (TNO), as well as PolyIC, a pioneer in printed organic electronics. The idea behind RFID (radio frequency identification) is unique, automatic identification of goods at the level of product or packaging units. The circuit, which Evonik produced with its three partners, is able not only to display data such as the standardized European Product Code (EPC) to a reading device but is actually able to communicate with the reader. This is useful for such applications as structured identification of various articles on a conveyor belt. The transmission rate of the chip is currently 10 kilobits per second and will increase to 25 kilobits per second by the end of the project. The RFID is currently working at the HF frequency of megahertz. By the end of the project, the plans are to achieve an RFID of 867 megahertz another novelty in the field of organic and large-area electronics. As with the solvent-based processing of oxidic semiconductors for thin-film transistors, the funded project was transferred to Evonik s Busi ness Unit Coatings & Additives in early E-PAPER WITH SOLUTION FROM CREAVIS E-paper prototype The prototype of the electronic paper from Toppan Printing, with thin-film transistors made of oxidic semiconductors from Evonik More than two years ago, the first demonstrator a prototype for electronic paper developed by the Toppan Printing Co., Ltd., one of the world s largest printing groups, headquartered in Tokyo showed just how well Evonik s method of solution-processing oxidic semiconductors for thin-layer transistors works. For this prototype, Toppan Printing produced the oxide semiconductor layer of the TFT from a solution-processable oxide semiconductor from Evonik. The material was applied by spin coating in a vacuum-free solution-based process, while other layers were fabricated using a standard vacuum depo sition process. Together, Toppan Printing and Evonik have lowered the processing temperature of the semiconductor to 270 C at that time a value 100 C lower than the known value for solution-processed oxide semiconductors. The on/off ratio was 10 5 at a charge carrier mobility of 0.5 square centi meters per volt and second, which is comparable to the charge carrier mobility and on/ off ratio of a conventional amorphous silicon TFT. Since then, the charge carrier mobility has been increased by a factor of 10 at this processing temperature.

22 PRINTED ELECTRONICS Solvent-based process for simple and cost-effective production of TFTs sputtering. In sputtering, energy-rich ions loosen individual atoms from a solid body.

This layers made of amorphous silicon can also be generated with chemical vapor deposition (CVD), in which a chemical reaction causes the deposition of silicon from the gas phase onto the

But new metal oxides eliminate this need.

This technology has a number of advan- While costly vacuum technology is currently used to apply functional layers, the materials from Evonik can simply be applied from solution.

5 22 PRINTED ELECTRONICS Solvent-based process for simple and cost-effective production of TFTs sputtering. In sputtering, energy-rich ions loosen individual atoms from a solid body. The atoms then depos it in a controlled way on the substrate that will hold the TFT backplane. This layers made of amorphous silicon can also be generated with chemical vapor deposition (CVD), in which a chemical reaction causes the deposition of silicon from the gas phase onto the surface of a heated substrate. Both processes, sputtering and CVD, work only in a vacuum environment, so display manufacturers must make a substantial investment in the equipment. But new metal oxides eliminate this need. Scientists at Creavis Science-to-Business Center Nano tronics have developed a procedure that allows the production of metal-oxide TFTs with a solventbased process. This technology has a number of advan- While costly vacuum technology is currently used to apply functional layers, the materials from Evonik can simply be applied from solution. In the future, a printing process can apply these materials directly and already structured, eliminating the need for costly photolithographic structuring Disposition (CVD or Sputter) Coating Printing Today Tomorrow After tomorrow The process steps that are currently required for conventional structuring of a semiconductor layer and that could be replaced by a single printing step in the future Resist coating Soft baking Semiconductor Substrate (SiOx) UV-Exposure Cleaning Developing Resist-stripping Wet-etching

6 PRINTED ELECTRONICS 23 tages over sputtering or CVD. First, the coating process does not require a vacuum environment, which reduces the amount of investment in the plant to a small part of the amount required for a CVD plant. Second, deposition from the liquid is relatively easy to scale: Simply put, the only work to do is to add another coating nozzle. But those are not the only advantages. Even now, a manufacturer can be prepared for the future of printed electronics with deposition from the liquid phase: Innovative solventbased semiconductor inks are used in both the processing of the semiconductor from a liquid, as well as in printing the transition from vacuum coating to solvent-based coating, therefore, appears to be a logical step on the path to high-resolution printing of electronic circuits in the future. More importantly, for the flexible displays of the future, manufacturers will have to replace the glass substrate currently used with a plastic film. Most of today s semiconductor materials, however, require temperatures far higher than 250 C for processing. With the solvent-based metal oxides from Creavis, on the other hand, processing temperatures can be set so low that even plastic films can be used as substrates another advantage that makes the new technology a good investment in the future. 333 New process suitable for plastic substrates Measurement data for a thin-film transistor. Each transistor in the flat screen works like an electrical switch and controls the brightness of the pixel. High current in the on state and a high ratio between the on and off states of the transistor are key to fast image buildup and good picture contrast. The Evonik materials meet both of these criteria Drain current [ampere] OFF state Switching range ON state Measurement set-up for contacting of a test substrate 1 m 100 µ 10 µ 1 µ 100 n 10 n 1 n 100 p 10 p 1 p Gate voltage [volt]

24 PRINTED ELECTRONICS FROM SCIENCE TO BUSINESS Moving from Creavis to Coatings & Additives The transfer of the Printed Electronics project from Creavis to the Coatings & Additives Business Unit on

7 24 PRINTED ELECTRONICS FROM SCIENCE TO BUSINESS Moving from Creavis to Coatings & Additives The transfer of the Printed Electronics project from Creavis to the Coatings & Additives Business Unit on January 1, 2012, was a first for Evonik. It marked the first time a business area that was developed as part of the Science-to- Business (S2B) concept was transferred, together with all the em ployees and research and application technology, to a business unit. For Creavis, the transfer of this innovation project is a great success, says Dr. Harald Schmidt, head of Creavis. As a strategic research and development unit, it is our job to establish new business for Evonik and develop futureoriented technology platforms. Printed Electronics is an impres sive example of that work. Dr. Ulrich Küsthardt, head of the Coatings & Additives Business Unit, stresses: We are completely convinced that, with our expertise in coatings on the one hand and experience in building businesses on the other, no one is better able to place the business segment in this future-oriented market and develop it further. We can see ourselves opening up completely new fields of application for the use of coating systems and additives in the electronics industry. Established in 2005, Nanotronics was Creavis first S2B Center. The S2B Center Nanotronics was also home to the Low-Cost Flexible Solar Cells and Smart Coatings projects, in addition to the Printed Electronics project. The Solar project is continuing within Creavis. External partners are utiliz ing the results of the Smart Coatings project formulations, know-how, customer contacts. License negotiations are currently underway with various companies. In addition, Creavis still maintains the S2B Centers Biotech - nology and Eco², which focus on white biotechno l ogy and energy efficiency and climate protection, respectively, as well as the Advanced Project House Light & Elec tronics in Taiwan, and the Project House Systems Integration in Hanau, which has completed its operational phase. All projects of this project house are now transferred to the business units.

PRINTED ELECTRONICS 25 Characterization of thin-film transistors 333 For the liquid process, the scientists from the Science-to-Business Center Nanotronics have found paths that enable the production

In addition, it was important that the inks do not show any unwanted sedimentation of solid materials.

8 PRINTED ELECTRONICS 25 Characterization of thin-film transistors 333 For the liquid process, the scientists from the Science-to-Business Center Nanotronics have found paths that enable the production of homogeneous amorphous metal-oxide layers with as few inner boundaries as possible following the application and then evaporation of the solvent. In addition, it was important that the inks do not show any unwanted sedimentation of solid materials. This is crucial to produce extremely thin layers on surfaces several meters long on the sides, where the characteristics of the thin-film transistors deviate from each other by only a few percentage points under operating conditions. In January 2012, Evonik s Coatings & Additives Business Unit transferred the entire Printed Electronics project from Creavis, including the equipment and roughly 20 employees, to its R&D unit a step the corporation had never before taken on this scale. The current business plan provides for convert ing the development into a business over the next few years, with the potential to become a product line or even a business line. To this end, the Elec tronic Solutions team is working together with several of the world s leading display manufacturers. The next step is to adapt the development to the process-specific requirements of the manufacturers. For Evonik, the business would be a completely new market for which it has held nothing in its product portfolio until now. 777 Dr. Ralf Anselmannn is head of the Electronic Solutions unit in the Coatings & Additives Business Unit since January 1, Anselmann studied chemistry at the University of Kaiserslautern, where he earned his doctorate in He then began his career at Merck KGaA in Darmstadt in the Pigments division. Beginning in 1988, he spent five years at the production site in Savannah (Georgia, USA) for the purpose of establishing a local research and engineering unit for pigments. After returning to Germany, he held various positions in R&D and technical marketing in Darmstadt. Then, in 2001, he moved from operative responsibility for R&D/AT Cosmetic Pigments to Central Business Development Chemistry, where he was responsible for the business development of the nanomaterials of Merck KGaA. In 2004, Anselmann moved from Merck to the former Degussa to establish and head the Science-to-Business Center Nanotronics based on the projects Printed Electronics, Low-Cost Flexible Solar Cells, and Smart Coatings , ralf.anselmann@evonik.com Dr. Jürgen Steiger is responsible for Printed Electronics in the Coatings & Additives Business Unit since January 1, He studied physics and material science in Freiburg, London, Heidelberg, and Darmstadt. After earning his doctorate in 2001, he initially worked for over three years in the Organic Light- Emitting Diodes unit of a start-up company in Frankfurt am Main. There, his work focused primarily on the inkjet printing of polymers for displays. He was also responsible for technical support for various display manu facturers. In 2004, he moved to the Printed Electronics project of Creavis Science-to-Business Center Nano tronics. He has been head of the project since He completed a part-time MBA program in , juergen.steiger@evonik.com

the Most Popular Display Technology?

Why is LCD the Most Popular Display Technology? History of Liquid Crystal Display (LCD) As early as 1889, scientists discovered that chemicals such as cholesteryl benzoate, when melted into liquid form,