Emerging Flexible Electronics Technology

　　Flexible electronics is an emerging science and technology. Due to its unique flexibility and extensibility, flexible electronics has broad application prospects in many areas.

　　Flexible electronics (Flexible Electronics), also known as plastic electronics (Plastic Electronics), printed electronics (Printed Electronics), organic electronics (Organic Electronics), and polymer electronics (Polymer Electronics), is an emerging electronic technology that fabricates electronic devices from organic/inorganic materials on flexible/stretchable plastic or thin metal substrates.

　　In people's impression, organic materials such as plastics are good insulators, and few would think that plastics can conduct electricity. In recent years, due to breakthroughs in conductive polymer research, organic materials have transformed from traditional insulators into conductive semiconductors, giving rise to flexible electronics. The development of modern chemistry and other technologies has promoted the growth of this discipline. The key to flexible electronics manufacturing includes manufacturing processes, substrates, and materials, with the core being micro- and nano-patterning manufacturing, involving interdisciplinary research in mechanics, materials, physics, chemistry, and electronics.

　　Flexible electronics, with its unique flexibility/extensibility and efficient, low-cost manufacturing processes, has broad application prospects in fields such as information, energy, healthcare, and defense, including flexible electronic displays, organic light-emitting diodes (OLED), printed radio-frequency identification (RFID), thin-film solar panels, electronic newspapers, and electronic skin (Skin Patches)/artificial muscles.

　　Flexible electronics integrates technologies from electronic circuits, electronic components, materials, flat panel displays, and nanotechnology, while also spanning industries such as semiconductors, packaging and testing, materials, chemical engineering, printed circuit boards, and display panels. It can assist traditional industries like plastics, printing, chemical engineering, and metal materials in transformation and upgrading, thereby enhancing industrial added value. Therefore, the development of flexible electronics technology will inevitably bring revolutionary changes to industrial structures and human life.

　　Flexible electronics technology is a brand-new electronic technology revolution that has attracted worldwide attention and developed rapidly. The American journal Science listed advances in organic electronics as one of the top ten scientific achievements of 2000, alongside major discoveries such as the human genome draft and cloning technology. American scientists Alan Heeger, Alan MacDiarmid, and Japanese scientist Hideki Shirakawa were awarded the 2000 Nobel Prize in Chemistry for their pioneering work in conductive polymers.

　　Differences between flexible electronics and traditional electronic manufacturing

　　Currently, the electronics industry is basically traditional semiconductor industry, with manufacturing equipment being very large, costly, and inefficient. The concept of flexible electronics aims to replace traditional semiconductor products, components, and circuits with printed methods. The differences between flexible electronics and traditional electronic circuits can be viewed from three aspects:

　　(1) Application prospects

　　Once very soft substrates are applied in design or circuits are made invisible or foldable, it differs greatly from traditional rigid substrates.

　　(2) Manufacturing cost

　　Flexible electronics uses roll-to-roll printing processes and can avoid wasting over 95% of materials as in photolithography. The printed area equals the used area, with utilization rates over 90%. From a long-term development perspective, printing methods are much cheaper than traditional photolithography. Silicon CMOS wafers typically cost $10 per square centimeter, compound semiconductors even more expensive, while the ideal cost for flexible electronics is $0.1 per square centimeter, showing its huge cost advantage.

　　(3) Investment perspective

　　Traditional semiconductor factories require investments of tens or even hundreds of billions, but flexible electronics printing is like traditional printing, where tens of millions can establish a basic scale. It should be emphasized that the inks used for printing differ from traditional printing inks and require special development. Initial development costs are higher due to small volumes, but costs decrease significantly with mass production.

　　Structure and materials of flexible electronic systems

　　Although flexible electronics technology can be applied in different fields, its basic structure is similar, including at least the following four parts: electronic components, flexible substrate (flexible substrate), interconnects (interconnect), and adhesive layer.

　　Structure of flexible electronic systems

　　1. Electronic components

　　Electronic components are the basic parts of flexible electronic products, including commonly used thin-film transistors, sensors, etc. These electronic components are essentially no different from those in traditional electronics. Some components use inorganic semiconductor materials (such as silicon), which are brittle and prone to fracture during deformation. Therefore, they are usually not directly placed on the circuit board but first mounted on rigid micro-cell islands, which are then distributed on the flexible substrate. This protects the electronic components from damage during bending. Of course, some electronic components can be directly distributed on the flexible substrate; for example, some thin-film transistors can directly withstand certain strains without affecting their function due to their characteristics.

　　Compared with traditional microelectronics technology, the use of organic electronic components is a significant feature in flexible electronics technology. Organic thin-film transistors play a very important role, and the use of organic materials creates conditions to reduce component weight and thickness and improve flexibility and extensibility.

　　2. Flexible substrate

　　The flexible substrate is the most prominent difference between flexible electronics technology and traditional electronics technology. It shares common characteristics with traditional rigid substrates, first being insulation: insulated flexible substrates ensure that electronic devices do not leak electricity during use, ensuring normal operation and safety. Secondly, high strength: in any electronic technology, the substrate acts as a skeleton, and without sufficient strength, normal use cannot be guaranteed.

　　Thirdly, low cost: substrate materials are among the most used materials in circuits, and only by using inexpensive materials can the cost of electronic products be effectively reduced.

　　In addition to the common characteristics of substrates mentioned above, flexible substrates have their own unique properties. First is flexibility: the flexibility of flexible electronic systems is mainly manifested through the substrate. Products with different flexibility requirements can use substrates made of different materials; for example, electronic skin usually uses highly flexible silicone resin (Silicone), while flexible electronic displays have lower flexibility requirements than electronic skin and mostly use polyethylene terephthalate (PET), commonly known as polyester.

　　Secondly, the film characteristic: although called a substrate, its size is no longer a "plate" but a film; flexible electronic system substrates are usually about 1 millimeter thick, which reduces material costs and lightens the product's weight.

　　Considering the above, using polymer polymers for flexible substrates is an ideal choice. Currently available flexible substrate materials include DuPont's Kapton polyimide (PI) film material, polydimethylsiloxane, polyethylene terephthalate (PET), etc., all of which can well meet insulation, flexibility, and strength requirements.

　　3. Cross-linked Conductors

　　Electronic components are first distributed on rigid microcell islands, many such microcell islands are then distributed on the flexible substrate. These microcell islands do not exist independently; they are connected by cross-linked conductors, forming a complete flexible circuit. In other words, cross-linked conductors act as wires in flexible electronic systems. The cross-linked conductors are attached to the flexible substrate in the form of metal films.

　　4. Adhesive Layer

　　The bonding of various components in flexible electronic systems requires an adhesive layer, which is especially important for bonding cross-linked conductors and flexible substrates. The adhesive layer in flexible electronic systems should have the following characteristics:

　　(1) Heat resistance. Flexible electronic products inevitably experience environments above room temperature during assembly and use, so a certain degree of heat resistance is necessary.

　　(2) Bonding strength. Since flexible electronic products undergo continuous tensile, compressive, and bending deformation during use, and the two thin layers connected by the adhesive layer usually have different mechanical properties, insufficient bonding strength will inevitably lead to relative sliding or even delamination of the two thin layers.

　　(3) Bending capability. The adhesive layer itself is a component of the flexible electronic system structure, and its own bending capability significantly affects the bending capability of the entire structure. Common adhesive layer materials used in flexible circuits currently include acrylic resin and epoxy resin.

　　5. Cover Layer

　　The cover layer (also called the encapsulation layer) mainly protects the flexible circuit from dust, moisture, or chemical corrosion, and also reduces the strain the circuit endures during bending. Recent studies show that the cover layer can reduce the stress intensity at the edges of rigid microcell islands (fcellisland) in flexible circuits and can inhibit their delamination from the flexible substrate.

　　According to the characteristics of flexible electronic systems, the cover layer needs to withstand long-term flexing. Therefore, the cover layer material, like the substrate material, must meet certain fatigue resistance requirements. Additionally, the cover layer covers the circuit after sub-etching, so it requires good conformability to meet the requirement of bubble-free lamination. Common materials used for the cover layer include acrylic resin, epoxy resin, and polyimide.

　　Fabrication Process of Flexible Electronic Systems

　　Like traditional IC technology, manufacturing processes and equipment are the main driving forces for the development of flexible electronics technology. The performance indicators of flexible electronics manufacturing technology include chip feature size and substrate area size. The key is how to manufacture flexible electronic devices with smaller feature sizes on larger substrates at lower costs.

　　The flexible electronics manufacturing process usually includes: material preparation → deposition → patterning → encapsulation, and can be integrated through roll-to-roll (R2R) substrate transport.

　　Flexible electronics manufacturing mainly focuses on production cost, production efficiency, achievable feature size, and compatibility of organic materials. In recent years, due to breakthroughs in active materials and their patterning technologies, flexible electronics manufacturing technology has made significant progress.

　　The core of flexible electronics manufacturing is thin-film transistor (TFT) fabrication. The key manufacturing technology is high-resolution patterning of the channel length between source and drain, which directly affects device performance such as output current and switching speed. During organic semiconductor patterning, it is especially necessary to eliminate parasitic leakage and reduce crosstalk to ensure a high on/off ratio. Most applications require organic thin-film transistor (OTFT) channel lengths less than 10 micrometers. Existing patterning technologies include photolithography, shadow masking, and printing (microcontact printing and inkjet printing).

　　Energy beam technologies such as photolithography are widely used in microelectronic device patterning with high resolution, but due to complex processes, expensive equipment, solvents and developers being unsuitable for plastic substrates, and time and material consumption, they are only suitable for small-area patterning. During etching of the underlying layer, environmental requirements are stringent, and removing photoresist damages the activity of organic electronic materials and polymer substrates, limiting their application in flexible electronics manufacturing.

　　Shadow mask technology is a "dry" process that avoids solvent damage to organic semiconductors but has limited resolution.

　　Printing technology simultaneously achieves functional material deposition and patterning in one step. The main methods are: (1) transferring and pasting the complete circuit onto a flexible substrate, such as transfer printing (stamping); (2) directly fabricating the circuit on the flexible substrate, such as inkjet printing and microcontact printing (soft lithography).

　　In traditional printing methods, the entire structure is first fabricated on a silicon wafer or glass plate using standard photolithography, then transferred to a flexible substrate to produce high-performance devices. Due to the use of photolithography and high-temperature deposition technologies, transfer printing can only manufacture small-area devices and has high processing costs.

　　Microcontact printing can produce multi-level patterns for masks and can be integrated with R2R mass production technology. Typically, one master can produce more than 100 stamps, each stamp can make over 3000 imprints, and the cost of stamps is relatively low. It can produce 60nm high-resolution patterns at speeds of several centimeters per second, but multi-layer patterning is difficult. Microcontact printing can be used for amorphous silicon, polycrystalline silicon, TMOS, and other materials but is difficult to use directly for organic material etching. Lan Hongbo and others have provided detailed discussions and analyses on the research progress and development trends of nanoimprint etching mold technology.

　　The ideal patterning process for flexible electronics should meet: low cost, large area, mass production, low temperature, additive, non-contact, real-time adjustability, three-dimensional structuring, easy multilayer alignment, printable organic/inorganic materials, etc. Inkjet printing is a non-contact, pressure-free, and plate-free printing replication technology. It features plate-less digital printing, digitally writing solutions at room temperature to achieve flexible digital printing, simplifying the manufacturing process. Using solution-processed semiconductor and metal materials instead of traditional vacuum deposition materials can effectively reduce costs. Inkjet printing also has the following advantages:

　　(1) Pattern quality is not limited by photolithography focal length and can be patterned on non-flat surfaces and even deep trench structures;

　　(2) Good compatibility with organic/inorganic materials;

　　(3) Direct processing of devices using CAD/CAM data enables large-area dynamic alignment and real-time adjustment;

　　(4) As a non-contact patterning technology, it can effectively reduce defects and use virtual masks to compensate for interlayer deformation, misalignment, and other defects;

　　(5) On-demand printing (DOD) technology without physical masks;

　　(6) Enables rapid design and processing of complex three-dimensional microstructures, and allows quick graphic changes through software-based print control systems.

　　Applications of Flexible Electronics

　　With the development of flexible electronics technology, various electronic products have emerged. Just as microelectronics technology provides a technical platform for large-scale integrated circuits and computer chip technology, flexible electronics technology offers a brand-new technical platform for new product development. Flexible electronic products are currently in the early stages of research and development, with some products already on the market. According to current R&D trends, flexible electronics technology has broad applications in the following three areas.

　　1. Flexible Electronic Displays

　　Flexible electronic displays are brand-new products developed on the flexible electronics technology platform. Unlike traditional flat-panel displays, these displays can be repeatedly bent and folded, bringing great convenience to our lives.

　　For example, all visual materials, including various books, newspapers, magazines, and video files, can be presented through this display and viewed anytime and anywhere. Although devices like MP4 players and personal digital assistants (PDAs) can meet such usage needs, their screens cannot be bent or folded and only allow reading and viewing within a small screen area, greatly limiting the visual experience. In contrast, flexible electronic displays have incomparable advantages; they are like newspapers that can be unfolded when needed and rolled or folded after use, ensuring portability while fully considering visual effects.

　　It is worth mentioning that flexible electronic displays use more lightweight organic materials instead of inorganic materials, making them lighter than traditional displays, which enhances portability. Additionally, the use of polymer organic materials offers the possibility of cost reduction. Moreover, flexible electronic displays are thin, with thickness far less than currently popular liquid crystal displays, so another name for flexible electronic displays is paper-like electronic displays.

　　2. Thin-Film Solar Panels

　　Thin-film solar panels are another specific application of flexible electronics technology. In today's world, energy has become a globally focused issue, and China faces not only energy shortages but also environmental pollution. Solar energy, as a clean energy source, can effectively alleviate energy shortages without environmental pollution.

　　As the most common way to utilize solar energy, solar panels can cover large areas at the lowest cost to effectively harness solar energy. Currently, amorphous silicon thin-film solar panels have been developed and entered the market.

　　Thin-film solar panels based on flexible electronics technology can meet high-power generation needs, such as use in solar power plants in sunny desert areas.

　　Besides, their flexibility and light weight allow integration into clothing. Wearing such clothes while walking or exercising in the sun can power portable small devices (like MP3 players and laptops) through the thin-film solar panels on the clothing, achieving both energy saving and environmental protection.

　　3. Application Background of Flexible Electronics in RFID Field

　　Radio Frequency Identification (RFID) technology features contactless information input and processing, fast and convenient operation, and rapid development, widely used in production, logistics, transportation, medical care, food, and anti-counterfeiting fields. RFID systems usually consist of transponders and readers.

　　Electronic tags are one form of transponders, understood as thin-film structured transponders, characterized by ease of use, small size, light weight, and embedment within products. In future RFID systems, electronic tags will be increasingly used.

　　The structure of electronic tags is evolving towards being lighter, thinner, smaller, and softer. In this regard, flexible electronic devices have unmatched advantages over other materials. Therefore, the future development of RFID electronic tags is likely to combine with flexible electronics manufacturing, making RFID tags more widely used and convenient. Additionally, this can greatly reduce costs and bring higher benefits. This is also one of the future development directions of flexible electronics manufacturing.

　　Producing low-cost flexible electronic tags has two significances. On one hand, it is a beneficial attempt to manufacture flexible electronic devices. Electronic circuits and devices are developing towards being "light, thin, small, and soft," and the development of flexible electronic circuits and devices is particularly notable.

　　For example, flexible circuit boards that can now be produced contain delicate wiring and are made of thin, flexible polymer films. They are compatible with surface mount technology and can be bent into countless required shapes.

　　Flexible circuits using SMT technology are very thin and lightweight, with insulation thickness less than 25 microns. These flexible circuits can be bent arbitrarily and even bent to fit inside cylinders, fully utilizing three-dimensional volume.

　　This breaks the traditional fixed usage area mindset, enabling full utilization of volume and shape, greatly increasing effective usage density compared to current methods, forming high-density assembly forms. This aligns with the trend of "flexibilization" in electronic products.

　　On the other hand, it can accelerate the recognition and development of RFID technology in China. In RFID systems, transponders are the key technology. Electronic tags are one form of RFID transponders, and flexible electronic tags are suitable for more occasions. Reducing the cost of electronic tags will strongly promote the widespread application of RFID technology.

　　4. Electronic Skin

　　Another important application of flexible electronics technology is electronic skin. Also called skin-like electronics, its basic feature is integrating various electronic components on a flexible substrate to form a skin-like circuit board with high flexibility and elasticity, usable in many other electronic devices.

　　For example, electronic skin can be widely applied in robotics: it integrates various sensors and conductors, converting external force or heat into electrical signals transmitted to the robot's computer for processing. Therefore, electronic skin is also called sensitive skin.

　　The two most fundamental characteristics of electronic skin can be summarized as:

　　(1) Having flexibility and elasticity like human skin so that robots can move flexibly and agilely like real people.

　　(2) Sensors are distributed on the electronic skin so that robots can sensitively perceive changes in the external environment.

　　Currently, research on electronic skin is flourishing both domestically and internationally. Regarding the important electronic components of robot skin—sensors—domestic research has made certain progress in their principles and applications. The design method of electronic skin based on infrared sensors has improved the robot's ability to perceive unknown environments to avoid obstacles in time.

　　In addition, the problem of signal fusion among many sensors in electronic skin has been solved. Since PVDF piezoelectric film has high piezoelectric capability, flexibility, extreme thinness, and light weight, closely resembling human skin characteristics, research on electronic skin sensors based on this material is relatively common domestically and abroad.

　　Foreign research on issues related to electronic skin has also made considerable progress. Japanese researchers have not only developed related theories of electronic skin but also manufactured experimental products. They studied the force sensing problem of electronic skin and explored the possibility of force sensing by establishing the relationship between force and sensor capacitance.

　　Although the basic principle of electronic skin is not complicated, covering a robot with electronic skin is quite challenging because electronic skin must perceive the robot's entire external environment and must be integral. At the same time, as an external component, electronic skin is susceptible to damage from external factors. When the electronic skin is damaged wholly or partially, timely replacement is necessary.

　　To meet this demand for electronic skin, the concept of electronic skin unit modules was proposed, connecting each unit module through a serial bus to achieve both integrity and scalability of the electronic skin. However, the use of wires inevitably increases the weight of the electronic skin, and the insulating rubber layer of the wires also somewhat limits the flexibility of the electronic skin.

　　Under these circumstances, the conductive connections of electronic skin have been deeply studied, and a new technology was proposed where a metal (gold) thin film is attached to a pre-stretched polyethylene terephthalate, commonly known as polyester substrate. Experiments show that this metal film remains conductive even under a stretching deformation of up to double its length (i.e., strain reaching 100%).

　　Research on electronic skin sensors currently mostly focuses on sensing a single external signal (such as force). However, as electronic skin, sensing multiple external signals simultaneously is very important, meaning it can simultaneously sense force, temperature, humidity, and other external stimuli. To achieve this goal, technical breakthroughs are needed in at least three areas:

　　(1) Material selection: The realization of sensor sensing functions depends to some extent on the functional characteristics of sensor materials, such as piezoelectricity, pyroelectricity, or semiconductivity, so research and application of functional materials influence the development of sensor technology.

　　(2) Processing of multiple sensitive signals: A complete (robot) electronic skin gathers a considerable number of sensing microelements, each capable of responding to the external environment. In some cases, each microelement must respond to multiple signals simultaneously (such as sensing force and heat at the same time). When the signal volume is quite large, how to process the signals and then determine the robot's response to external stimuli is an important issue.

　　(3) Optimization of the mechanical properties of electronic skin: As an important application of flexible electronic technology, electronic skin must achieve flexibility while ensuring strength; optimizing for the lightest possible mass without causing damage is also an important consideration.

　　Besides robots, electronic skin can also be applied in artificial organs, such as artificial hearts used to treat heart disease, which naturally imposes more stringent requirements on the materials of electronic skin. In summary, electronic skin fully utilizes the lightweight and flexible characteristics of flexible electronic systems and has broad application prospects.

　　With the development of flexible electronic technology, various electronic products have emerged. Just as microelectronic technology provides a technological platform for large-scale integrated circuits and computer chip technology, flexible electronic technology offers a brand-new technological platform for the development of new products.