Evolving Flexible Electronics

- Nov 29, 2019-

Evolving Flexible Electronics

Summary of content: Flexible electronic technology is an emerging science and technology. Due to its unique flexibility and ductility, flexible electronic technology has broad application prospects in many aspects. Flexible electronics (Flexible) technology is an emerging science and technology. Due to its unique flexibility and ductility, flexible electronic technology has broad application prospects in many aspects.

Flexible Electronics is also known as Plastic Electronics, Printed Electronics, Organic Electronics, Polymer Electronics, etc .; it is the production of organic / inorganic materials electronic devices in flexible / Emerging electronics on ductile plastic or thin metal substrates.

In people's impression, organic materials, such as plastic, are good insulators, and few people think that plastic can also conduct electricity. In recent years, due to new breakthroughs in the research of conductive polymers, organic materials can change from traditional insulators to conductive semiconductors, and flexible electronics have emerged at the historic moment. The development of modern chemistry and other technologies has promoted the development of a discipline such as flexible electronics. The key to flexible electronics manufacturing includes manufacturing processes, substrates, and materials. The core of this is micro- and nanopatterning manufacturing, which involves cross-disciplinary research in machinery, materials, physics, chemistry, and electronics.

With its unique flexibility / ductility and efficient and low-cost manufacturing processes, flexible electronics have a wide range of applications in the fields of information, energy, medical and defense, such as flexible electronic displays, organic light emitting diodes OLED, printed radio frequency identification (RFID), Thin-film solar panels, electronic newspapers, and electronic skin patches / artificial muscles.

In addition to integrating electronic circuits, electronic components, materials, flat display and nanotechnology, flexible electronics also span semiconductor, packaging, testing, materials, chemical, printed circuit board and display panel industries, which can assist traditional industries such as The transformation of industries such as plastics, printing, chemicals and metal materials will increase the added value of the industry. Therefore, the development of flexible electronic technology will bring revolutionary changes to the industrial structure and human life.

Flexible electronic technology is a brand-new electronic technology revolution, which has attracted wide attention from the world and has developed rapidly. The United States "Science" magazine listed the progress of organic electronics technology as one of the world's top ten scientific and technological achievements in 2000, juxtaposed with major discoveries such as the human genome sketch and cloning technology. American scientist Allen Haig, Allen Mark Dilmide, and Japanese scientist Hideki Shirakawa won the 2000 Nobel Prize in Chemistry for their pioneering work in the field of conductive polymers.

The difference between flexible electronics and traditional electronics manufacturing

At present, the electronics industry basically belongs to the traditional semiconductor industry. The equipment used for manufacturing is quite large, the cost is high, and the manufacturing efficiency is low. The whole concept of flexible electronics is to hope that traditional semiconductor products, components and circuits can be printed by printing. Instead. There are three main differences between flexible electronics and traditional electronic circuits:

(1) Application Prospect

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 rigid substrate.

(2) Manufacturing costs

Flexible electronics uses a roll-to-roll printing process, and the use of materials can also avoid the problem of waste of more than 95% of materials such as photolithography. The area printed by printing is equivalent to the area used, and its utilization rate is Above 90%, from the perspective of long-term development, the printing method will be much lower than the cost of traditional photolithography; the general cost of silicon CMOS wafers is $ 10 per square centimeter, compound semiconductors are even more expensive, and the ideal cost for flexible electronics is US $ 0.1 per square centimeter, the huge advantages of flexible electronics can be seen from the cost.

(3) Investment perspective

Traditional semiconductor factories need billions or even tens of billions of investment at every turn, but the way of flexible electronic printing is like traditional printing. As long as the investment is tens of millions, the basic scale can be established. It should be emphasized that the ink used for printing is different from traditional printing and requires special development. The initial cost of development is relatively high due to the small amount, but the cost will become lower after mass production.

Structure and materials of flexible electronic systems

Although flexible electronic technology can be applied to different fields, its basic structure is similar and includes at least the following four parts: electronic components, flexible substrates, interconnects, and adhesive layers.

Flexible electronic system structure

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 are not fundamentally different from those of traditional electronic technology. Some components use inorganic semiconductor materials (such as silicon). Because of their brittle material, they are prone to brittle fracture during deformation, 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 cell islands that carry the components are then distributed on the flexible substrate. The advantage of this is to protect the electronic components and avoid them. Damage during bending. Of course, some electronic components can also be directly distributed on the flexible substrate, for example, some thin film transistors can directly withstand certain strains without affecting their functions due to their own characteristics.

Compared with traditional microelectronic technology, the use of organic electronic components is a significant feature in flexible electronic technology. Among them, organic thin film transistors occupy a very important position. The use of organic materials is to reduce the weight and thickness of components. Increasing its flexibility and ductility creates the conditions.

Flexible substrate

Flexible substrate is the most prominent point of flexible electronic technology different from traditional electronic technology. It has the common characteristics of traditional rigid substrates. The first is insulation: the flexible insulating substrate guarantees that electronic equipment will not leak electricity during use, which not only ensures its normal operation but also its safety in use. The second is higher strength: no matter what kind of electronic technology, the role of the substrate is equivalent to the role of the skeleton. Without high strength to guarantee, normal use cannot be guaranteed.

Once again, it is cheap: the substrate material is one of the most used materials in the circuit. Only the use of inexpensive materials can effectively reduce the cost of electronic products.

In addition to the common characteristics of the above substrates, flexible substrates have their own unique characteristics. The first is flexibility: the flexibility of the flexible electronic system is mainly expressed by the substrate. Products with different requirements for flexibility can use substrates of different materials; for example, electronic skins usually use very flexible silicon organic resin (Si1icone), and flexibility Electronic displays have weaker requirements for electronic skin than electronic skins, and most commonly use polyethylene terephthalate (PET), commonly known as polyester.

Secondly, thin film: Although it is called a substrate, it is no longer a "board" in size, but a thin film; the substrate of a flexible electronic system is usually about 1 mm, which reduces the cost of materials and reduces the product's weight.

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

3. Crosslinked Conductor

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

4. Adhesive layer

The bonding of various components of a flexible electronic system requires an adhesive layer, and the adhesive layer is particularly important for the combination of a crosslinked conductor and a flexible substrate. The adhesive layer of a flexible electronic system should have the following characteristics:

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

(2) Cohesion. Because flexible electronic products are constantly subjected to tensile, compression, and bending deformation during use, the two thin layers connected by the adhesive layer usually have different mechanical properties. If the bonding force is not large enough, it will inevitably lead to the relative sliding of the two thin layers or even Peel off.

(3) Bending ability. 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 are mainly acrylic resin and epoxy resin.

5. Overlay

The cover layer (also known as the encapsulation layer) mainly protects the flexible circuit from dust, moisture or chemicals, and can also reduce the strain on the circuit during bending. Recent research shows that the cover layer can reduce the flexibility of the circuit. The stress intensity of the edge of the rigid microcell island can suppress its delamination from the flexible substrate.

According to the characteristics of the flexible electronic system, the covering layer needs to be able to withstand long-term deflection, so the covering layer material is the same as the substrate material, and 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 good conformability to meet the requirements of bubble-free lamination. Common materials used for the cover layer are acrylic resin, epoxy resin, and polyimide.

Preparation process of flexible electronic system

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

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

Flexible electronics manufacturing focuses on factors such as production cost, production efficiency, achievable feature size, and compatibility of organic materials. In recent years, due to the breakthroughs in active materials and their patterning technology, flexible electronics manufacturing technology has developed significantly.

The core of flexible electronics manufacturing is thin film transistor (TFT) manufacturing. The key manufacturing technology is high-resolution patterning technology for the channel length between source and drain, which directly affects device performance such as output current and switching speed. In the organic semiconductor patterning 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 to be less than 10 microns. Existing patterning technologies include photolithography, shadow masks, and printing (micro-contact printing and inkjet printing).

Energy beam technologies such as photolithography are widely used in the patterning of microelectronic devices with high resolution, but because of their complicated process, expensive equipment, solvents and developers cannot be used for plastic substrates, plus time-consuming and expensive materials, they are only suitable for Small area patterning, harsh environment requirements when etching the bottom layer, removal of photoresist will destroy the activity of organic electronic materials and polymer substrates, etc., which is limited in flexible electronics manufacturing applications.

The shadow mask technology is a "dry" process that prevents solvents from damaging organic semiconductors, but has limited resolution.

Printing technology achieves functional material deposition and patterning in the same step at the same time. The main methods are: (1) transfer and paste the complete circuit to a flexible substrate, such as printing (stamp); (2) directly on the flexible substrate Preparation of circuits such as inkjet printing and micro-contact printing (soft etching).

In the traditional printing method, the entire structure is first prepared on a silicon wafer or glass plate by a standard photolithography method, and then transferred to a flexible substrate to produce a high-performance device. Due to the application of photolithography and high-temperature deposition technology, transfer printing technology can only manufacture small-area devices, and the processing cost is high.

Micro-contact printing can produce multi-level patterns for masks, which can be integrated with R2R batch manufacturing technology. Generally, a master can make more than 100 stamps, and each stamp can achieve more than 3,000 stamps. The cost of stamps is relatively low. 60nm high-resolution patterns can be produced at a speed of several centimeters per second, but multilayer patterns are realized. more difficult. 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 it for etching organic materials. Lan Hongbo and others made a detailed discussion and analysis on the research progress and development trend of nanoimprint etch mold technology.

The ideal patterning process for flexible electronics should meet: low cost, large area, batch process, low temperature, "plus", non-contact, real-time adjustment, three-dimensional structure, easy multilayer registration, printable organic / inorganic Materials, etc. Spray printing is a non-contact, pressure-free, and printing-free printing reproduction technology. It has the characteristics of plateless digital printing. Direct solution writing at room temperature enables digital flexible printing, which simplifies the manufacturing process. The use of solutionized semiconductor and metal materials instead of traditional vacuum deposition materials can effectively reduce costs. Printing also has the following advantages:

(1) The quality of the pattern is not limited by the focal length of the lithography, and it can be patterned on non-planar surfaces and even deep groove structures;

(2) Good compatibility with organic / inorganic materials;

(3) Directly use CAD / CAM data to process devices, which can realize large-area dynamic alignment and real-time adjustment;

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

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

(6) The rapid design and processing of complex three-dimensional microstructures can be realized, and the graphics can be quickly changed through a software-based print control system.

Applications of flexible electronics

With the development of flexible electronic technology, various electronic products have emerged at the historic moment. Just as microelectronics technology provides a technology platform for large-scale integrated circuits and computer chip technology, flexible electronic technology provides a new technology platform for the 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. From the current research and development trends, flexible electronic technology has a wide range of applications in the following three areas.

Flexible electronic display

Flexible electronic display (flexible electronic display) is a new product developed on the flexible electronic technology platform. Unlike traditional flat panel displays, this type of display 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 viewed anytime, anywhere. Although things like MP4 players and personal digital assistants

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