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Making chips smaller, more powerful and more integrated is the unremitting pursuit of scientific and technological workers. Professor Wei Dacheng's team from the State Key Laboratory of Polymer Molecular Engineering, Department of Polymer Science, Fudan University, designed a new type of semiconductor photoresist with excellent performance. Using photolithography technology, 27 million organic transistors were integrated and interconnected on a full-frame chip. From 100,000 in 2021 to 27 million today, the team has made continuous breakthroughs in the integration of polymer semiconductor chips in recent years, leading the world to reach a level of ultra-large-scale integration, and providing important support for the further practical application of organic chips.

Highly densely interconnected organic transistor arrays on flexible substrates

Drawing on silicon-based chip lithography technology, the integration of organic chips is increased to tens of millions

When people talk about "chips" in daily life, they mostly refer to silicon-based chips, which are semiconductor chips made of single-crystal silicon and are widely used in the fields of computers and communications. Organic chips, made of organic materials such as polymer semiconductors and conjugated small molecules, have the advantages of intrinsic flexibility, biocompatibility, and low cost, and have important application prospects in emerging fields such as wearable electronic devices and bioelectronic devices.

With the development of modern information technology, the integration density of functional chips is getting higher and higher. The density of silicon-based chip integrated devices has exceeded 200 million transistors per square millimeter. In comparison, organic chips lag far behind silicon-based chips in terms of integration and reliability.

Chip integration can be divided into small-scale integration (SSI), medium-scale integration (MSI), large-scale integration (LSI), very large-scale integration (VLSI) and ultra-large-scale integration (ULSI), and the number of monolithic integrated devices is greater than 2, 26, 211, 216 and 221 respectively.

According to previous public reports, the highest integration level of polymer semiconductor chips has reached the level of large-scale integration (LSI). For example, in 2021, a foreign team produced the highest stretchable transistor array density, integrating more than 10,000 elastic transistors on an area smaller than a thumb (0.238 cm2).

Is it possible to further improve the integration of organic chips? Today, Wei Dacheng's team has given the answer - they have designed a functional photoresist and used photolithography technology to integrate 27 million organic transistors on a full-frame chip and achieve interconnection, reaching the level of ultra-large-scale integration (ULSI).

"We have made a breakthrough in the traditional organic chip processing technology." Wei Dacheng introduced that, unlike silicon-based chips, the manufacturing methods of traditional organic chips mainly include screen printing, inkjet printing, vacuum evaporation, photolithography, etc., and they borrowed the photolithography technology of silicon-based chips to increase the integration of organic chips to the tens of millions level.

(a) Photoresist composition; (b) Photoresist aggregation structure; (c) Organic transistor arrays processed on different substrates; (d) Schematic diagram of the organic transistor array structure and optical microscope photos; (e) Pixel density comparison of organic phototransistor imaging chip (PQD-nanocell OPT) with existing commercial CMOS imaging chips and organic imaging chips manufactured by other methods.

The key to photolithography technology lies in photoresist. Photoresist, also known as photoresist, plays an important role in chip manufacturing. It can transfer the required fine patterns from the mask to the substrate to be processed through processes such as exposure and development. It is a basic material for photolithography.

Traditional photoresists are only used as processing templates and do not have the functions of conductivity and sensing. They need to be cleaned after use. However, the new functional photoresist developed by Wei Dacheng's team forms a nanoscale interpenetrating network structure after photocrosslinking. It has good semiconductor performance, photolithography processing performance and process stability. It can not only realize the reliable manufacture of sub-micron feature size patterns, but the pattern itself is a semiconductor, which simplifies the chip manufacturing process.

The photoresist can achieve different sensing functions by adding sensing receptors. In order to achieve highly sensitive photodetection, the team loaded core-shell nanoparticles with photovoltaic effect into the photoresist material. Under light, the nanophotovoltaic particles generate photogenerated carriers, and the electrons are captured by the core, resulting in in-situ grating regulation, which greatly improves the light response of the device.

The results were published in Nature Nanotechnology on July 4 under the title "Photovoltaic nanocells for high-performance large-scale-integrated organic phototransistors".

Five years of interdisciplinary research to overcome the core difficulties in organic chip manufacturing

Since 2018, Wei Dacheng's team has embarked on the journey of developing semiconductor photoresists, and he himself has been engaged in the research of organic semiconductor materials since his graduate school. "If a work is to achieve a real breakthrough, it must take a long time of accumulation." He said that the team not only tried different materials and structures, but also accumulated rich practical experience.

Wei Dacheng, a professor in the Department of Polymer Science, said that the successful development of functional photoresists is inseparable from an interdisciplinary research team. Team members must not only master professional knowledge such as chemical synthesis and material science, but also overcome professional barriers and learn to apply knowledge such as electronic device design and manufacturing.

Wei Dacheng takes a photo with students

"We need to understand many issues, such as how to design and synthesize high-performance organic semiconductor materials, how to accurately construct electronic devices through photolithography technology, and how to optimize device structure to improve performance." In Wei Dacheng's view, this interdisciplinary collaboration model requires teachers and students to constantly learn new knowledge and jointly face and solve various problems.

During the research and development process, one of the major difficulties faced by the team was the structural design of the aggregated state of functional photoresists. The different functions of photoresists often affect each other. For example, the realization of the photocrosslinking function may destroy the conductive channel and cause a decrease in electrical performance. Through careful design and in-depth research on the structure-activity relationship, the team finally ensured that the photoresist can be cross-linked while having good conductivity and process stability, and has excellent overall performance.

Another major challenge is the standardized manufacturing of devices. "This link requires repeated exploration, and we have experienced many failures." Wei Dacheng admitted that the team started from scratch, accumulated experience through various experiments, and mastered the key technologies of organic chip design and manufacturing. In terms of hardware, the research and development of electronic devices also requires specific equipment and experimental conditions.

The organic phototransistor array prepared by the team on a 6-inch wafer

The development and optimization of electronic devices is a complex and delicate process. "Every detail cannot be ignored, because it is directly related to the overall performance of the device. In the future, we will continue to design the circuit layout to ensure that it can perform specific functions and meet actual application needs." Wei Dacheng said.

After going through many tests, the team's organic chip manufacturing level has made breakthroughs. As early as 2021, the density of integrated devices of polymer semiconductor chips developed by Wei Dacheng's team has reached 100,000 transistors per square centimeter. Today, the organic transistor interconnect array manufactured by lithography they have developed contains 4500×6000 pixels, with an integration density of 3.1 million transistors per square centimeter, and 27 million devices are integrated on a full-frame chip, reaching ultra-large-scale integration (ULSI), which is at the international leading level.

Rich and diverse application prospects, highly compatible with semiconductor industry production lines

"The birth of organic chips does not mean that they will replace silicon-based chips, but that they can play a unique role in specific fields." Wei Dacheng emphasized that the unique properties of organic semiconductor materials can serve as a supplement to current silicon-based chips and play a key role in certain fields.

Compared with single-crystalline silicon, the properties and functions of organic semiconductors can be customized through controlled synthesis, showing remarkable flexibility. It is undeniable that silicon-based chips still dominate in high-performance applications such as signal processing, especially in some high-end fields, where silicon-based chips are still the first choice.

"In actual application scenarios, diverse needs have given rise to diverse solutions. For innovative applications such as wearable devices, brain-computer interfaces, electronic skin, etc. with special application requirements, organic chips have demonstrated unique value. By carefully designing the molecular structure, we can give it diverse functional properties, enabling it to achieve functions or applications that silicon-based materials do not have." he said.

The advantage of organic semiconductors lies not only in their good flexibility, but also in their ability to achieve biocompatibility through structural regulation, thereby better adapting to the human environment.

(a, b) Schematic diagram of the structure of the human eye and bionic retina; (c) Demonstration of photoelectric synapse performance on a 5 × 5 transistor array; (d) Performance comparison of the bionic retina and traditional CMOS photodetectors in a neural network-based image recognition algorithm.

For example, one of the bionic electronic applications presented by Wei Dacheng's team at the end of the paper - the flexible retina not only has the same pixel density as the photoreceptor cells of the human retina, but also has similar memory effects and image processing functions. By imitating the adaptability of the human eye, this technology can provide solutions for visual aids and medical implants that are closer to the physiological characteristics of the human body, heralding a new direction for future bionic technology.

In the field of flexible display, taking the common organic light-emitting diode (OLED) as an example, it is precisely because of the application of organic small molecule materials that the bendable and foldable characteristics of the screen are realized, giving rise to the popular foldable screen mobile phones. The team's technology is also applicable to the pursuit of thin, light and bendable next-generation flexible display technology and drive circuits.

Currently, the team is actively seeking opportunities for cooperation with the industry to realize the transformation of scientific research results. Because the technology uses photolithography technology, it is highly compatible with the existing microelectronics industry. This means that large-scale production can be achieved on the existing silicon-based process line, thereby greatly reducing the threshold for industrialization.

"Customized research and development based on market demand will be the key to commercializing scientific research results." Wei Dacheng believes that this technology has broad prospects in promoting industrial upgrading and meeting major national needs, and organic chips will complement silicon-based chips and are expected to further promote the diversified development of microelectronics technology.