Detailed Analysis of Flexible Electronics Materials and Applications



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This article is a detailed explanation about flexible electronics. Flexible electronic technology is a new science and technology. The emerging electronic technologies built on flexible and ductile substrates are known as flexible electronic technologies. Due to its unique flexibility and extensibility, flexible electronic systems have broad application prospects in many aspects.

CoreFlexible electronicsPurposeAnalyze flexible electronics materials and applications
NamePlastic electronics,printed electronics,organic electronics,polymer electronicsCategoryGeneral electronics
ApplicationsFlexible electronic display,thin film solar panels,RFID,electronic skinFeatureUnique flexibility/extensibility



Catalogs

CatalogsI、What is Flexible electronics?3.Cross-linked polymer1.Flexible electronic display
II、The difference between flexible electronics and traditional electronics manufacturing4.Adhesion layer2.Thin film solar panels
III、Structure and materials of flexible electronic systems5.Cover layer3. RFID
1.Electronic componentsV、Flexible electronic system preparation process4.Electronic skin
2.Flexible substrateVI、Application of flexible electronics



Introduction

I、What is Flexible electronics?

Flexible Electronics is also called Plastic Electronics, Printed Electronics, Organic Electronics, Polymer Electronics, etc. It is to manufacture organic/inorganic material electronic devices in flexible/ New electronic technology on ductile plastic or thin metal substrates.

In people's minds, organic materials, such as plastics, are good insulators, and few people think that plastics can also conduct electricity. In recent years, due to new breakthroughs in the research of conductive polymers, organic materials can be transformed from traditional insulators to conductive semiconductors, and flexible electronics have emerged. The development of modern chemistry and other technologies has promoted the development of such a discipline as flexible electronics.

The key to flexible electronic manufacturing includes manufacturing processes, substrates, and materials. Its core is Micro- and Nanopatterning manufacturing, which involves cross-disciplinary research in machinery, materials, physics, chemistry, and electronics.

With its unique flexibility/stretchability and high-efficiency and low-cost manufacturing process, flexible electronics has a wide range of applications in information, energy, medical, defense and other fields, such as flexible electronic displays, organic light-emitting diodes, OLED, printed RFID, thin-film solar panels. , e-newspapers, skin patches/artificial muscles, etc.

Flexible electronics

Applications of flexible electronics

In addition to integration of electronic circuits, electronic components, materials, flat displays, nanotechnology, and other fields, flexible electronics can also assist traditional industries such as plastics in industries such as semiconductors, packaging and testing, materials, chemicals, printed circuit boards, and display panels. The transformation of industries such as printing, chemical, and metal materials will increase the added value of industries. Therefore, the development of flexible electronic technologies will inevitably bring revolutionary changes to the industrial structure and human life.

Flexible electronic technology is a brand-new revolution in electronic technology that has attracted worldwide attention and has been rapidly developed. The United States "Science" magazine listed organic electronic technology as one of the world's top ten scientific and technological achievements in 2000, alongside major human genome sketches, Cologne technology and other major discoveries. American scientists Allen Heuger, Alan Mark Demidity, and Japanese scientist Hideyuki Shirakawa received the 2000 Nobel Prize in Chemistry for their pioneering work in the field of conductive polymers.

What is flexible electronics and how does it work?



Detail

II、The difference between flexible electronics and traditional electronics manufacturing

The difference between flexible electronics and traditional electronics manufacturing

Difference between flexible electronics and traditional electronics 

At present, the electronics industry is basically a traditional semiconductor industry. The equipment used for manufacturing is huge, expensive, and has low manufacturing efficiency. The whole concept of flexible electronics is to use traditional semiconductor products, components, and circuits to print. Alternative. The difference between flexible electronics and traditional electronic circuits is mainly from three aspects:

(1) Application Prospects

Once a very soft substrate is applied to the design or the circuit is made invisible or foldable, it is very different from the traditional hard substrate.

(2) Manufacturing cost

The roll-to-roll printing process is used, and the use of materials can also avoid the problem of waste of more than 95% of material like lithography. The area printed by printing is equivalent to the area used, and its utilization rate is 90%. Above, from a long-term development perspective, the printing method will be much lower than the cost of traditional lithography; silicon CMOS wafers generally cost 10$/cm2, and composite semiconductors are even more expensive. The ideal cost for flexible electronics is 0.1$/ Cm2, the great advantage of flexible electronics can be seen from the cost.

(3) Investment perspective

The traditional semiconductor factories need to invest billions or even billions of dollars at a time, but the printing method is just like the traditional printing. If you invest tens of millions, you can establish a basic scale. It should be emphasized that the ink used for printing is different from traditional printing, and it needs to be specially developed. The initial cost of development is relatively low due to the small amount, but the cost after mass production will become lower.

III、Structure and materials of flexible electronic systems

Although flexible electronic technology can be applied in different fields, its basic structure is similar, including at least the following four parts: electronic components, flexible substrate, cross-linked conductive body, and adhesive layer.

Flexible electronic system structure

Flexible electronic system structure

The following sections describe the four major parts of the structure of a flexible electronic system.

  • 1.Electronic components

Electronic components are the basic components of flexible electronic products, including thin film transistors and sensors commonly used in electronic technology.

These electronic components and the traditional electronic technology components are not essential differences, some of the components using inorganic semiconductor materials (such as silicon), due to its relatively brittle material, in the deformation process is prone to brittle fracture, so they are usually not directly distributed in the circuit On the board, it is first placed on a rigid cell island, and then the microcell islands carrying the components are redistributed on the flexible substrate. This has the advantage of protecting the electronic components and avoiding them. Damaged during bending. Of course, some electronic components can also be directly distributed on flexible substrates, such as some thin-film transistors. Due to their own characteristics, they can directly withstand a certain strain without affecting their functions.

Compared with traditional microelectronics technology, the use of organic electronic components is a prominent feature in flexible electronic technology, in which the organic thin film transistor (OTFT) occupies a very important position, the use of organic materials. To reduce the weight and thickness of the components and increase their flexibility and ductility create the conditions.

  • 2.Flexible substrate

Flexible substrates are the most prominent place for flexible electronic technologies that differ from traditional electronic technologies. It has the common characteristics of the traditional rigid substrate, the first is the insulation: insulating flexible substrate to ensure that the electronic device does not leak during use, not only to ensure that it can work properly, but also to ensure the safety of its use.

The second is higher intensity: no matter which kind of electronic technology, the role of the substrate is equivalent to the role of the skeleton, without a higher strength to protect, it can not guarantee its normal use.

Again, it is cheap: The substrate material is one of the most used materials in the circuit, and only the use of low-cost materials can effectively reduce the cost of the electronic product.

In addition to the common features of the above substrates, flexible substrates also have their own unique characteristics. The first is flexibility: The flexibility of the flexible electronic system is mainly manifested by the substrate, and the substrates with different materials can be used for products with different requirements for flexibility; for example, the electronic skin is usually made of a very flexible silicone organic resin (Si1icone), and the flexibility is Electronic displays are less flexible than electronic ones, and polyethylene terephthalate (PET) is commonly used as polyester.

Followed by film: Although known as the substrate, but its size is no longer a "board", but the film; flexible electronic system substrate is usually about 1mm, which reduces the cost of the material, but also reduces the weight of the product .

In view of the above considerations, the use of high-molecular polymers for flexible substrates is the ideal choice. Currently available flexible substrate materials include DuPont Kapton Polyimide (PI) film materials, polydimethylsiloxane, polyethylene terephthalate (PET), etc. Can well meet the insulation, flexibility and strength requirements.

  • 3.Cross-linked polymer

The electronic components are first distributed on rigid micro-cell islands. Many of these micro-cell islands are redistributed on flexible substrates. These micro-cell islands do not exist independently. They are connected by cross-linked electrical conductors and thus form A complete flexible circuit, that is to say a cross-linked polymer, acts as a wire in a flexible electronic system. The crosslinked electrical conductor is attached to the flexible substrate in the form of a metal thin film.

  • 4.Adhesion layer

The combination of various components of a flexible electronic system requires an adhesive layer, and the adhesive layer is particularly important for the combination of a cross-linked electrical conductor and a flexible substrate.

The adhesive layer of the flexible electronic system should have the following characteristics:

(1) Heat resistance

During the assembly and use of flexible electronic products, it is unavoidable to experience an environment higher than normal temperature, and certain heat resistance is necessary.

(2) binding force

Since the flexible electronic products are constantly subjected to tensile and compressive bending during use, the two thin layers connected by the adhesive layer generally have different mechanical properties. If the bonding strength is not enough, the relative sliding of the two thin layers will inevitably result. Peel off.

(3) Flexibility

The adhesive layer itself is an integral part of the structure of the flexible electronic system, and its own bending ability has an important influence on the bending ability of the entire structure. At present, the adhesive layer materials commonly used in flexible circuits mainly include acrylic resin and epoxy resin.

  • 5.Cover layer

The cover layer (also called encapsulation layer) mainly protects the flexible circuit from dust, moisture, or chemicals and also reduces the strain experienced by the circuit during bending. Recent studies have shown that the cover layer can reduce the flexibility of the circuit. The stiffness of the edge of the rigid microcell island (fcellisland), and its delamination with the flexible substrate can be suppressed.

According to the characteristics of the flexible electronic system, it is required that the cover layer can tolerate long-term flexing. Therefore, like the cover material and the substrate material, the fatigue resistance must meet certain requirements. In addition, the cover layer covers the circuit after the sub-etching, so it is required to have a good conformability to meet the bubble-free lamination requirements. Common materials for the cover layer are acrylics, epoxies, and polyimides.

V、Flexible electronic system preparation process

Like traditional IC technologies, manufacturing processes and equipment are also the main driving forces for the development of flexible electronic technologies. The level of flexible electronics manufacturing technology includes chip feature size and substrate area. The key is how to manufacture flexible electronic devices with smaller feature sizes at a lower cost on larger-format substrates.

Flexible electronic manufacturing processes typically include: Material Preparation → Deposition → Patterning → Packaging, which can be integrated by roll-to-roll (R2R) substrate transport.

R2R technology-based flexible electronic manufacturing process diagram

R2R technology-based flexible electronic manufacturing process diagram

Flexible electronic manufacturing focuses on factors such as 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 electronic manufacturing technologies have achieved significant development. .

The core of flexible electronic manufacturing is thin film transistor (TFT) manufacturing. The key manufacturing technology is to create a high-resolution patterning technology for the channel length between source and drain, directly affecting the device performance such as output current and switching speed. Patterning of organic semiconductors During the process, it is particularly necessary to eliminate parasitic leakage and reduce crosstalk to ensure a high switching ratio. Most applications require organic thin film transistor (OTFT) channel lengths of less than 10 μm. Current patterning techniques include photolithography, shadow masks, Print (micro contact printing and jet printing) etc. Specific comparisons are shown in the table below.

Lithography and other energy beam technologies have been widely used in the patterning of microelectronic devices. The resolution is high, but due to the complex process, expensive equipment, solvent and developer can not be used for plastic substrates, and time-consuming materials, only applicable to Small area patterning requires harsh environmental conditions when etching the underlying layer. Degradation of photoresist can destroy the activity of organic electronic materials and polymer substrates, and is limited in flexible electronic manufacturing applications.

Shadow mask technology is a “dry” process that avoids solvents destroying organic semiconductors but has limited resolution.

The printing technology simultaneously deposits and patterns functional materials in the same step. The main methods are: (1) Transfer and affix a complete circuit to a flexible substrate, such as printing (stamp); (2) Directly on a flexible substrate Prepare circuits such as jet printing and microcontact printing (soft etching).

In the transfer method, the entire structure is first fabricated on a silicon wafer or a glass plate by a standard photolithography method, and then transferred to a flexible substrate to manufacture a high-performance device. Since photolithography and high-temperature deposition techniques are applied, transfer printing technology can only be manufactured. Small area devices, and high processing costs.

Micro-contact printing can produce multi-level patterns for masking, which can be integrated with R2R batch manufacturing technology. Usually one master can make more than 100 stamps, and each stamp can realize more than 3,000 stamps, stamps The cost is relatively low, 60 nm high-resolution patterns can be produced at speeds of several centimeters per second, but it is difficult to realize multi-layer patterns. Micro contact printing can be used for a variety of materials such as amorphous silicon, polysilicon, and TMOS, but it is difficult to directly use In the organic material etching. Lan Hongbo et al. On the nanoimprint etching mold technology research progress and development trend of the detailed discussion and analysis.

The ideal patterning process of flexible electrons should satisfy: low cost, large area, batch process, low temperature, “plus” type, non-contact, real-time adjustment, three-dimensional structure, easy multilayer registration, printable organic/inorganic Materials, etc. As can be seen from the table above, jet printing is a non-contact, pressure-free, non-printing printing and copying technology. It has the characteristics of digital printing without printing, and the solution is directly written at room temperature to realize digital flexible printing. Simplify the manufacturing process. The use of solubilized semiconductors and metal materials to replace traditional vacuum deposition materials can effectively reduce costs, and jet printing also has the following advantages:

(1) Pattern quality is not limited by the lithography focal length and can be patterned on non-planar surfaces or even deep trench structures

(2) Good compatibility with organic/inorganic materials;

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

(4) As a non-contact patterning technology, defects can be effectively reduced, and virtual masks can be used to compensate defects such as deformation and misalignment between layers;

(5) Print on demand (DOD) technology without physical masking;

(6) It can realize the rapid design and processing of complex 3D micro-structures, and can quickly change the graphics through a software-based print control system.



Analysis

VI、Application of flexible electronics

With the development of flexible electronic technology, various electronic products have emerged. Just as microelectronics technology provides a technology platform for large-scale integrated circuits and computer chip technologies, flexible electronic technology provides a brand-new technological platform for the research and development of new products. Flexible electronic products are currently in the initial stage of research and development, and some products have been put on the market. Judging from the current R&D trends, flexible electronics has a wide range of applications in the following three areas.

  • 1.Flexible electronic display

The flexible electronic display is a brand-new product developed on the flexible electronic technology platform. Unlike traditional flat-panel displays, such displays can be repeatedly bent and folded, thus bringing great convenience to our lives.

For example, all visual materials, including books, newspapers, magazines, and video files, can be presented on this display and can be viewed anytime, anywhere. Although current popular MP4 players and personal digital assistants (PDAs) can meet such use needs, the display screen cannot be bent and folded, and can only be read and viewed in a small screen area. And video, visual effects are greatly constrained. In contrast, flexible electronic displays have unparalleled advantages. They are like newspapers. When they are needed, they are unfolded. When they are used, they are curled or even folded. This guarantees the convenience of portability while giving full consideration to the visual effects.

Flexible electronic display

Flexible electronic display

Samples of flexible electronic displays have been successfully developed and it is believed that it will be a long way from entering the market. It is worth mentioning that flexible electronic displays use more lightweight organic materials instead of inorganic materials, so their weight is lighter than traditional displays, and this feature helps to improve their portability. In addition, the use of high molecular organic materials offers possibilities for reducing costs. In addition, the flexible electronic display has the characteristics of a thin thickness, and its thickness can be much smaller than that of the popular liquid crystal display. Therefore, another name of the flexible electronic display is a paper-like electronic display.

  • 2.Thin film solar panels

Thin film solar cel1 is another specific application of flexible electronics technology. In today's world, energy has become a topic of global concern. China not only faces energy shortages but also faces environmental pollution. As a clean energy source, solar energy can effectively alleviate the contradiction of energy shortage under the premise of zero environmental pollution.

As the most common way to use solar energy, solar panels can cover a large area at the lowest cost to effectively use solar energy. At present, thin film amorphous Sili-Con solar panels have been successfully developed and marketed.

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

In addition, it can also make full use of its flexible and lightweight features to integrate it into clothing. Putting on such clothes to walk or exercise in the sun, the power of small appliances (such as MP3 players and laptops) that are carried around can be supplied by the thin-film solar panels on the clothes, thus achieving the purpose of saving and environmental protection.

Thin film solar panels

Thin film solar panels

  • 3.RFID

Radio frequency identification (RFID) technology can be used to complete information input and processing, fast and convenient operation, and rapid development without manual contact, and is widely used in production, logistics, transportation, medical, food, security and other fields. RFID systems usually consist of transponders and readers.

The electronic tag is one of many forms of transponder, and can be understood as a transponder with a thin film structure, which has the characteristics of convenient use, small size, thin and light, and can be embedded in the product. More and more electronic tags will be used in future RFID systems.

Application Background of Flexible Electronics in RFID Field

Application Background of Flexible Electronics in RFID Field

The structure of electronic tags has evolved toward light, thin, small and soft. In this regard, flexible electronic devices have unparalleled advantages over other materials, so the future development of electronic tags in RFID systems is likely to be combined with flexible electronic manufacturing, making the use of RFID electronic tags more extensive and convenient. In addition, it can also reduce costs to a large extent and bring higher benefits. This is also one of the directions for the future development of flexible electronics manufacturing.

Making low-cost flexible electronic tags has two meanings. On the one hand, it is a useful attempt to make flexible electronic devices. Electronic circuits and electronic devices are developing in the direction of “light, thin, small, and soft”, and the development of flexible electronic circuits and electronic devices is even more striking.

For example, a flexible circuit board that can be produced nowadays is a circuit that contains delicate wires and is made of a thin, compliant polymer film that can be applied to a surface mount technology and can be bent into an infinite number of desired shapes.

The flexible circuit using the SMT technology is thin, lightweight, and has an insulation thickness of less than 25 microns. This flexible circuit can be arbitrarily bent and can be bent into a cylinder to make full use of the three-dimensional volume.

It breaks the traditional fixed-use area of mindset, thus forming the ability to make full use of volumetric shapes, which can greatly increase the effective use density in the current methods used to form high-density assembly forms. Conform to the development trend of "flexibility" of electronic products.

On the other hand, it can accelerate the recognition and development of radio frequency identification technology in our country. In radio frequency identification systems, transponders are the key to technology. Electronic tags are one of many forms of RFID transponders, and flexible electronic tags are more suitable for more applications. The reduction in the cost of electronic tags will greatly promote the real application of radio frequency identification technology.

  • 4.Electronic skin

Another important application of flexible electronics is electronic skin. Electronic skin, also called skin-like electrons, is basically characterized in that various electronic components are integrated on a flexible substrate to form a skin-like circuit board, which has high flexibility and elasticity like skin and can be used in many other applications. electrical equipment.

For example, electronic skin can be widely used in robotics: electronic skin integrates various sensors and electrical conductors, converts external stress or heat conditions into electrical signals and transmits them to the robot's computer for signal processing. Sensitive skin.

Electronic skin

Electronic skin

The two most basic features of electronic skin can be summarized as:

1) It is as flexible and elastic as human skin so that the robot can behave like a human and be flexible and agile.

2) The sensor is distributed on the electronic skin so that the robot can feel the change of the external environment sensitively.

At present, research on electronic skin at home and abroad is in the ascendant. For the important electronic component of the robot's skin - the sensor, the domestic has made some progress in its principle and applied research. The design method of the electronic skin based on the infrared sensor improves the robot's ability to sense the unknown environment so as to avoid obstacles in time.

In addition, the problem of signal fusion for many sensors in electronic skin has been solved. Because PVDF piezoelectric films have high piezoelectricity, flexibility, thinness, and light weight, and are very close to the characteristics of human skin, research on electronic skin sensors around the material is common at home and abroad.

The foreign research on the related problems of electronic skin has also made considerable progress. Japanese researchers not only developed the theory of electronic skin, but also produced experimental products. Aiming at the force perception problem of electronic skin, the possibility of realizing force sensing is discussed by establishing the relation between force and sensor capacitance.

Although the basic principle of the electronic skin is not complicated, how to cover the electronic skin with the robot is quite challenging, because the electronic skin is the external environment of the robot body and must be integrated. At the same time, the electronic skin exists as a kind of appearance component. The possibility of damage due to external factors. When the whole or part of the electronic skin is damaged, it needs to be replaced in time.

For this kind of demand of electronic skin, the concept of electronic skin unit module was put forward, and each unit module was connected through a serial bus, thereby achieving the unity and scalability of electronic skin. However, the use of wires inevitably increases the weight of the electronic skin, and the insulating rubber layer of the wire also restricts the softness of the electronic skin to some extent.

Under this circumstance, an in-depth study was made on the connecting conductors of the electronic skin, and a new technique of attaching a metal (gold) film to pre-stretched polyethylene terephthalate, commonly known as a polyester substrate, was proposed. Experiments have shown that this metal film can still conduct electricity at up to one times tensile deformation (ie, strain reaches 100%).

Researches on electronic skin sensors are currently focused on the sensing of a single external information (such as force). However, as an electronic skin, sensing of multiple external information is very important, that is, external stimuli such as force, temperature, and humidity can be felt at the same time. To achieve this goal, at least three technological breakthroughs need to be achieved:

(1) Material selection: The realization of the sensor sensing function depends to a certain extent on the functional properties of the sensor material, such as piezoelectricity, pyroelectricity or semiconductivity, so the research and application of functional materials affect the sensor technology. development of.

(2) The processing of multiple sensitive signals: A complete (robot) electronic skin has a considerable number of sensory elements, each of which has a function of responding to the external environment. In some cases, each feels The microelements also respond to multiple signals (e.g., simultaneous susceptibility and heat) at the same time. In the case of a very large signal volume, how to process the signal and determine the robot's countermeasures against external stimuli is an important issue.

(3) Optimization of electronic skin mechanical properties: As an important application of flexible electronic technology, electronic skin must meet the realization of flexibility under the guarantee of strength; the lightest quality optimization design under the premise of no damage is also an important content to consider.

In addition to robots, electronic skin can also be used in artificial organs, such as artificial heart for heart disease, which of course imposes more stringent requirements on electronic skin materials. In short, the electronic skin has fully utilized the flexible and flexible structure of the flexible electronic system and has a wide application prospect.

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



Book Recommendation

Flexible Electronics: From Materials to Devices 

This book provides a comprehensive overview of the recent development of flexible electronics. This is a fast evolving research field and tremendous progress has been made in the past decade. In this book, new material development and novel flexible device, circuit design, fabrication and characterizations will be introduced. Particularly, recent progress of nanomaterials, including carbon nanotubes, graphene, semiconductor nanowires, nanofibers, for flexible electronic applications, assembly of nanomaterials for large scale device and circuitry, flexible energy devices, such as solar cells and batteries, etc, will be introduced. 

--by Guozhen Shen, Zhiyong Fan

Flexible and Wearable Electronics: Design and Fabrication Techniques

The objective of this book is to provide a comprehensive guide to the state of the art technologies and methods applied in the realization of flexible and wearable electronics. The targeted readers of this book include but not limited to college professors, Research and Development scientists, practicing electronics and material engineers, in addition to all the enthusiasts interested in modern technological advancements. Moreover, the book serves as an extensive resource for graduate students working on topics related to wearable and flexible electronics. This book, organized into eleven chapters, introduces the latest research findings and trends related to the design, fabrication processes and techniques used in the realization of wearable and flexible electronics.

--by Haider K Raad



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