While the conventional electronics like computers and smartphones is built around silicon integrating billions of transistors and is manufactured using complex, costly and wasteful processes in multi-billion dollar foundries . The printed and flexible electronics aim to replace this by “organic” semiconductors which are long chains of thousands of repeating molecules (a plastic), made with materials based on carbon. Organic semiconductors can be made to be soluble, and can be turned into an ink.
Printing electronics is a special type of manufacturing, where electronic components, circuits, and systems are developed on a wide variety of substrates in a similar fashion as drawing text and figures on a paper, textile, and handicrafts. The difference between normal printing and printed electronics is that in printed electronics, functional material is used as ink that exhibits functionalities of insulator, conductor, and semiconductor materials, which are essential for the electronic devices. This means it’s possible to print electronic circuits, with the potential to manufacture components as fast as printing newspapers. A printer would do this by applying different inks onto the film. As the inks dried, they would turn into wires, transistors, capacitors, LEDs and all the other things needed to make displays and circuits.
Simplified processing steps, reduced materials’ wastage, low-fabrication costs, and simple patterning techniques make printing technologies very attractive when compared to standard microfabrication in clean room processes. The potential benefits of printed and flexible electronics include thinness, lighter weight, greater durability, and the ability for conformal integration. The intrinsic scalability of printing as a manufacturing process is also of great advantage; the process lends itself to production runs of an arbitrary size. The concept appears to provide a likely path to truly all-embracing electronics. Printed and flexible electronics (PFE) have potential to revolutionize multiple markets—health care, environmental monitoring, displays, human-machine interactivity, energy, communication, and wireless networks.
The global printed electronics market size was almost $30 billion in 2017 and a growth potential to over $70 billion in ten years. The surging demand for the flexible electronics at low manufacturing costs and the need for eco-friendly technologies is paving the way for increased adoption of the technology. Additionally, the increasing penetration of IoT worldwide is proving to be a prime factor in pulling the printed electronics market over the forecast period. The continuously growing demand for IoT in the telecommunication industry for enhancing the network and optimizing the performance along with operations is expected to propel the application of technology over the forecast period.
PFE Challenges and manufacturing
In most of the fabrication of electronic devices, these materials are sprayed over the substrate with the help of printing technology as low as few nanometers thick (thin film) and few micrometers width (pattern). The combination of thin films and patterns can make any electronic device to be used in the electronic circuits. Although the process seems simple, but the limitations posed by the various parameters such as uniformly dispersed and stable colloidal solutions, substrate treatments, and above all, optimized printing recipes make the printing process much more challenging.
The prominent challenges in printed electronics are material compatibility, substrate surface energy, viscosity of the materials’ solution, and compatibility of dissimilar materials in multilayer structure, technology limitations in terms of film thickness, width, and height. Printed and flexible electronics have thus far failed to achieve widespread adoption due to significant unresolved technical challenges. Major gaps exist between expectations and performance of printed electronics in the areas of logic, memory, analog circuitry, power, and light generation. However Printed electronics have evolved substantially from the early implementations of basic conductive copper or silver traces printed on hard (and later flexible) substrates to which conventional electronic components were attached. Now, thanks to advances in materials science, printed electronics incorporates a wide variety of capabilities, from roll-to-roll memories to printable thin film transistor (TFT) logic and even wireless communications.
The different printing technologies based on contact/noncontact approach are summarized in Figure. In contact printing process, the patterned structures with inked surfaces are interfaced physically at controlled pressures with the target substrate. In noncontact process, the solution is dispensed through the openings or nozzles, and structures are defined by moving the substrate holder in a preprogrammed pattern. The contact-based printing technologies consist of gravure printing, gravure-offset printing, screen printing, flexographic printing, microcontact printing, nanoimprint, and dry transfer printing. The prominent noncontact printing techniques include slot-die coating, electrohydrodynamics, and inkjet printing.
The noncontact printing techniques have received greater attractions due to their distinct capabilities such as simplicity, affordability, speed, adaptability to the fabrication process, reduced material wastage, high resolution of patterns, and easy control by adjusting few process parameters
There are three important manufacturing techniques that are key enablers across this diversified industry: functional inkjet printing, thin film encapsulation and roll-to-roll processing.
The fascinating field of printed electronics is enabled by the rapid developments in printing technologies for precise deposition of functional inks in easy and cost-efficient way. The ultimate goal of developing the printed electronic technology is to revolutionize the device manufacturing and maximize the throughput by covering areas larger than wafer scales as well as increase the production through roll-to-roll processes. The attraction of printed electronic technology is that it can be executed at ambient conditions, thus enabling the fabrication of biocompatible electronics. With the help of printed electronic technologies and biocompatible materials, it is possible to fabricate electronic device on plastic substrates and even on human skin. The most commonly used and reliable printed electronic technologies are explained in the below sections.
It has been widely recognized that roll-to-roll processing of flexible devices can induce a breakthrough in cost reduction. For some of today’s flexible electronics products, the production volume is already high enough to justify roll-to-roll production. Organizations are now aggressively pursuing P/F application solutions that include, at minimum, signal processing hardware and microcontrollers, along with proprietary embedded software whose algorithms are major product differentiators that enable a systems solutions approach.
Noncontact printing technologies
Inkjet material printer
Inkjet printing has gained a significant interest in recent years for processing solution-based nanomaterials and patterning on diverse substrates in a single step. Nanoparticles of the functional materials are mixed in compatible solvents to prepare printable ink. Besides, chemical-based solutions are also prepared and adjusted to the jetting parameters of the inkjet printing systems. Materials are ejected in micrometer-sized droplets through miniaturized nozzle printheads. Mainly two mechanisms for the actuation of inkjet nozzle head have been developed, i.e., thermal and piezoelectric. Droplets in very small diameters are ejected at each corresponding pulse and generated by either thermal or piezoelectric actuators used in the inkjet nozzle head.
Inkjet printers are further divided into two types as continues inkjet (CIJ) and drop on demand (DoD) systems. In continuous inkjet printer, the ink is stored in a reservoir supplied to the nozzles. A charging electrode is used to pull ink out of the nozzle and form a droplet. After droplet formation, deflection plates are used to direct the droplets on the targeted area on the substrate. This kind of printing system is used in the industry for printing on the packages such as expiry date commonly seen on boxes and drinks of liquor bottles. As the droplets are not controlled by any input signal, the CIJ system continuously produces droplets with certain frequency; hence, the unused droplets are directed to the recycling system, where the ink is stored and reused.
On the other hand, DoD system is controlled by a digital input signal that comes from the design of the pattern. In DoD system, each droplet is generated on demand; hence, there is no need of deflection and charge electrodes.
Electrohydrodynamic (EHD) printing
Electrohydrodynamics (EHD) is another interesting type of inkjet printing systems used to deposit functional ink in the form of thin films as well as high-resolution patterns. It is consisted of high-speed camera, light source, nozzle and head, ink storage and supply mechanism, stage movement, and activity display unit as shown in Figure. Working principle of the EHD system is that the ink to be deposited on substrate is pumped from ink storage tank to nozzle with appropriate ink flow rate to make stable cone jet
Aerosol jet printing
Aerosol jet printing is an interesting technique in noncontact printing, which has attracted a significant interest in the manufacturing of high-resolution pattering. A wide variety of materials including insulators, semiconductors, and metallic conductors are processed in viscosity ranges of 1–1000 cps . The aerosol process is driven by the gas flows where a mist of microdroplets is generated as a result of pneumatic atomization or through ultrasonication. The capability to process a wide variety of materials and to pattern higher resolutions, as high as 10 μm on diverse substrates, makes aerosol jet printing most attractive among other contact-less printing techniques. Aerosol jet printing is divided into two main categories, i.e., pneumatic and ultrasonic, as shown in Figure. Both the techniques are based on different operational procedures and are used targeting specific set of requirements and goals
Slot die is a special deposition technique, where the material is printed on a moving substrate and installed directly on a roll-to-roll system, as shown in Figure The solution is directly dispensed off the slot-die printhead in a controlled manner. The slot-die coating is executed in two steps, where a uniform and stable flow of the coated material is achieved in the first phase. In the second phase, other processing parameters such as standoff distance between the slot-die opening and the target substrate speed of the rolling substrate and sintering conditions inline to promote a multilayer structure coating capability. Despite the high-speed coating capabilities, the challenges involved in reaching stable operating conditions make the process less attractive than other printing systems and are adopted seldomly in the manufacturing of printed electronic devices.
Contact-based printing technologies
In contact-based printing techniques, prepatterned structures of the printing tools are physically brought in conformal contact to the target substrate. Similarly, micron-scale dispensing nozzles are also used for high-resolution patterning by contacting the target surface in a similar fashion as drawing. Almost all the techniques used in contact-based printing are precise and rapid; therefore, these techniques are used by the industry for the mass productions.
Screen printing remains the top priority when it comes to rapid, fast, and large area manufacturing. The technology has been using from the early developmental stages of microelectronic industry, especially for printing electrodes and interconnections. Screen printing is advantageous when compared to other printing systems as it is more versatile, and processing is simple, capable of reproducing similar structures in large batches with minimum dimensional variations.
Screen printing has been successfully used for demonstrating various fabricated devices. For instance, an all-screen printing has been adopted for developing thin film transistors (TFTs). OLED devices are also presented by exploring the different process parameters, such as viscosity and mesh count, and their effect on the printed structures. An advanced screen-printing approach is adopted to develop multilayer high-density flexible electronic circuits connected through holes with embedded passive and optical devices. Printing interconnect lines between discrete devices on a same substrate or tape out for data reading are usually printed with screen printing.
Gravure printing is the most prominent and representative technique in the contact-based printing category. Structures are transferred through a prepatterned surface, where the solvent is deposited on the target surface upon contacting. The engraved structures are designed in cylindrical shaped objects, and substrate controlled through moving rolls of the system represents a typical R2R process. The gravure printing tools consist of a large cylinder electroplated with copper and engraved with microcells, as shown in Figure.
The limited time of contact and higher speed also enable high-resolution patterning on the substrate and increase reliability of the system. Reliability of gravure offset is more critical for assembling in a high-speed production line of printed electronics on rollable substrates
Flexographic printing ensures high-speed printing and produces high-resolution patterned structures as compared to gravure and gravure-offset printing approaches. A rubber- or polymer-based plate with elevated patterns on the surface and developed through photolithography is used in flexographic printing. The plate is attached to the printing cylinder as shown in Figure . A wide variety of ink including but not limited to solvent-based, wafer-based, UV-curable inks and two parts chemically curing inks, etc. can be processed to pattern high-resolution structures on target surfaces.
Microcontact printing (μCP)
Microcontact printing is a special type of contact-based printing approach, where an inked surface is brought in conformal contact and transfers the patterns on target surface. The contact is controlled through micromanipulation, and surface conditions are set to release and receive the ink consequently.
A master mold is developed using conventional microfabrication or photolithography techniques, and multiple copies of the stamp with desired structures are reproduces.
Nanoimprinting, as the name suggests, is used to produce structures at nanoscale by using an imprint approach. NI uses mechanical and physical deformation of wet layers through molding accompanied by different thermal procedures. The NI operating principles are quite straight forward, as shown in Figure. As against microcontact printing, NI uses a mold having nanoscale structures developed through standard clean room processes and is pressed against a uniformly coated wet surface at controlled pressure and temperature.
Transfer printing is relatively a new technique to fabricate flexible electronics through physical transfer of prefabricated structures using a stamp. Microstructures in the shape of wires or membranes are developed using standard photolithography processes in clean room, etched underway, and then used a stamp to pick and stamp on a target surface. Transfer printing can be executed in two ways such as direct transfer and stamp-assisted transfer. In direct transfer, an adhesive coating is performed on the receiving surface, and the donor wafer is directly contacted with it. After releasing, the structures are transferred to flipped surfaces on the target surface.
Mass production perspective
Developing printing technologies is motivated by the development of fast and efficient production line by assembling different manufacturing units, substrate treatments, and sintering after fabrication, as shown in Figure. The highly optimized techniques on lab level are merged together to develop a single manufacturing platform.
R2R as a commonly shared platform has the potential for a continuous and high-throughput process for deposition of diverse materials on large substrate rolls (often called “web”). Besides the instrumentation and hardware for control system, R2R line is equipped with several rollers over which the web (flexible substrates) passes with controlled tension.
Digital, additive manufacturing reduces cost
Inkjet printing of functional materials is an important enabler of cost reduction. Being an additive technology, it avoids the equipment, material and labor cost of additional processes related to traditional lithography – coating, exposure, artwork production and development. Next to this it adds flexibility in the production process due to its digital nature and offers miniaturization potential with its small picoliter droplet size. For these reasons inkjet is being explored and adopted in many different fields of the printed electronics market. Applications are found in virtually all areas of electronics manufacturing (logic and memory, displays, sensors, solar cells, PCB) and other industries (e.g. life sciences and pharmaceutics).
Packaging is an important step during electronic fabrication, which enables user to interface with electronic devices, circuits, and systems. In printed electronics, devices are relatively large size as compared to conventional technologies and easy to handle. However, they need packaging to protect from the ambient environment such as humidity, light, and temperature. On the other hand, the device itself needs protection from the user touches as it can damage the thin films and patterns. 3D printing (also called additive manufacturing) is a manufacturing technology, which is based on imposing the material layers to create the 3D objects.
A 3D object is fabricated through melting filament material with controlled temperature and flow rate in combination with X, Y, and Z axis control, as shown in Figure. The object is design in CAD tool, i.e., AutoCAD or any other tool that can create 3D structures and converted into printer supported file format. The file is then loaded into the printer to create the object in 3D form. There are several ways to create a 3D object, which defines the types of 3D printers.
Fused deposition modeling
The development of 3D object is made by either microdrops or melt near-field electrospinning of melted thermoplastics of consecutive layers, which solidifies after a certain time. Commonly used filament materials are PVA, PLA, ABS, nylon, and some composites. This 3D printing technology is often used in the rapid prototyping objects.
Stereolithography (SLA) uses a UV laser instead of melting the filament through heater and making microdrops. An SLA printer uses two mirrors in combination with UV laser, known as galvanometers, positioned on the X-axis and on the Y-axis. Both galvanometers rapidly aim a laser beam across a vat of resin, the area under light beam selectively curing and solidifying a cross section of the object inside this build area, building it up layer by layer and forming a 3D structure.
Digital light processing (DLP)
This 3D printing technology is almost the same as stereolithography, the only difference that instead of laser and two mirrors, DLP uses image of the 3D object to make one layer and repeat until the job is finished. DLP is much faster than SLA as it uses digital image array to produce a structure in the vertical sequential order frame by frame. The digital image of the target object is consisted of small rectangles called voxels. Potential applications of this technology are functional parts, complex ducting (hollow designs), and low run part production. This technology can be used for strong and elastic mechanical property parts and also for the complex geometry printing.
Selective laser sintering (SLS)
SLS 3D technology creates an object with powder bed fusion technology and polymer powder. Working of this technology, i.e., polymer powder, is preheated to a temperature slightly below the melting point. Then, a very thin layer of the powdered material is deposited with the help of a blade normally 0.1 mm thick on the object platform. The surface is then scanned with a CO2 laser beam, as it selectively sinters the powder and solidifies a cross section of the object according to the designed geometry. Same as SLA 3D technology, the laser is precisely focused on to the correct location with the help of two galvos.
Once the entire cross-sectional area of the object is scanned that creates one layer of the object, the build platform will move down one layer thickness in height to make the next step happen. The powder recoating blade deposits a new layer of the powder on the top of prescanned layer, and the laser will sinter the next cross section of the object area the same as previous layer. This process is continuous until the object is created.
3-D print electronics and cells printed directly on skin
In a groundbreaking new study, researchers at the University of Minnesota used a customized, low-cost 3D printer to print electronics on a real hand for the first time. Researchers also successfully printed biological cells on the skin wound of a mouse. The technique could lead to new medical treatments for wound healing and direct printing of grafts for skin disorders.
“We are excited about the potential of this new 3D-printing technology using a portable, lightweight printer costing less than $400,” said Michael McAlpine, the study’s lead author and the University of Minnesota Benjamin Mayhugh Associate Professor of Mechanical Engineering. “We imagine that a soldier could pull this printer out of a backpack and print a chemical sensor or other electronics they need, directly on the skin. It would be like a ‘Swiss Army knife’ of the future with everything they need all in one portable 3D printing tool.”
No matter how hard anyone would try to stay still when using the printer on the skin, a person moves slightly and every hand is different,” McAlpine said. “This printer can track the hand using the markers and adjust in real-time to the movements and contours of the hand, so printing of the electronics keeps its circuit shape.”
Another unique feature of this 3D-printing technique is that it uses a specialized ink made of silver flakes that can cure and conduct at room temperature. This is different from other 3D-printing inks that need to cure at high temperatures (up to 100 degrees Celsius or 212 degrees Fahrenheit) and would burn the hand.
To remove the electronics, the person can simply peel off the electronic device with tweezers or wash it off with water. In addition to electronics, the new 3D-printing technique paves the way for many other applications, including printing cells to help those with skin diseases. McAlpine’s team partnered with University of Minnesota Department of Pediatrics doctor and medical school Dean Jakub Tolar, an expert on treating rare skin disease. The team successfully used a bioink to print cells on a mouse skin wound, which could lead to advanced medical treatments for those with skin diseases. “I’m fascinated by the idea of printing electronics or cells directly on the skin,” McAlpine said. “It is such a simple idea and has unlimited potential for important applications in the future.”
Cost-effective method produces semiconducting films from materials that outperform silicon
Engineers have developed a technique to fabricate ultrathin semiconducting films made from a host of exotic materials other than silicon. To demonstrate their technique, the researchers fabricated flexible films made from gallium arsenide, gallium nitride, and lithium fluoride — materials that exhibit better performance than silicon but until now have been prohibitively expensive to produce in functional devices.
The new technique, researchers say, provides a cost-effective method to fabricate flexible electronics made from any combination of semiconducting elements, that could perform better than current silicon-based devices.
“We’ve opened up a way to make flexible electronics with so many different material systems, other than silicon,” says Jeehwan Kim, the Class of 1947 Career Development Associate Professor in the departments of Mechanical Engineering and Materials Science and Engineering. Kim envisions the technique can be used to manufacture low-cost, high-performance devices such as flexible solar cells, and wearable computers and sensors.
In 2017, Kim and his colleagues devised a method to produce “copies” of expensive semiconducting materials using graphene — an atomically thin sheet of carbon atoms arranged in a hexagonal, chicken-wire pattern. They found that when they stacked graphene on top of a pure, expensive wafer of semiconducting material such as gallium arsenide, then flowed atoms of gallium and arsenide over the stack, the atoms appeared to interact in some way with the underlying atomic layer, as if the intermediate graphene were invisible or transparent. As a result, the atoms assembled into the precise, single-crystalline pattern of the underlying semiconducting wafer, forming an exact copy that could then easily be peeled away from the graphene layer. The technique, which they call “remote epitaxy,” provided an affordable way to fabricate multiple films of gallium arsenide, using just one expensive underlying wafer.
“We found that the interaction through graphene is determined by the polarity of the atoms. For the strongest ionically bonded materials, they interact even through three layers of graphene,” Kim says. “It’s similar to the way two magnets can attract, even through a thin sheet of paper.”
Kim envisions that remote epitaxy can now be used to fabricate ultrathin, flexible films from a wide variety of previously exotic, semiconducting materials — as long as the materials are made from atoms with a degree of polarity. Such ultrathin films could potentially be stacked, one on top of the other, to produce tiny, flexible, multifunctional devices, such as wearable sensors, flexible solar cells, and even, in the distant future, “cellphones that attach to your skin.” “In smart cities, where we might want to put small computers everywhere, we would need low power, highly sensitive computing and sensing devices, made from better materials,” Kim says. “This [study] unlocks the pathway to those devices.”
Achieving Printed Power Electronics Means Going Beyond Silver Nanoparticles
“The same properties that make silver appealing for printed electronics—that ability to sinter at low temperatures because it’s very unstable and the atoms migrate with low temperatures and low energy—this can lead to premature failures when you have higher current densities running through it,” said Greg Fritz, a material scientist in the Charles Stark Draper Laboratory. “So much of the technology of printed electronics has been on printed, lower power circuits that work fine with silver nanoparticle inks, but Draper has products that are high power and quite small so we are not able to use the printed silver for these products.”
Because of this, Fritz and his team at Draper are currently working on alternative techniques for printing high power electronics. “We are finding that drone boards are actually pretty high power so we are trying to print the ‘guts’ of the drone. Between the battery and the rest of the drone— the lights and the coms and the fan and all the other things—there is one board that gets all the power. It can require a single amp or more than ten amps of current, and that’s a lot of power for silver to endure when you scale down the size of the conductor.”
We are also looking at high power antennas. We have 3D antennas that we are making now with wire or copper or other bulky materials, but it would be really nice if we could just print these materials, and today we can’t do that with the available printed inks.
What we’ve come up with is a way to have this tradeoff. We start off with a material that is nano-layered—so it is unstable as an ink, it has a nano-spaced reactants. Once we print the ink and we sinter it, we are able to form a new material, which is an alloy of the layered structure we started off with, but now this new alloy has completely different properties. It is inherently high-temperature stable. We’ve demonstrated this, and we’ve made conductors this way.
The downfall of this is that we need to keep improving the other properties—the mechanical stability for flexible electronics, the electronic connectivity to make it more directly competitive with the available silver. However, we’ve been able to make these new inks and we are currently printing them in all kinds of commercial printers. We are off to a promising start.
Hurdle of Encapsulation
“Globally, the main problem of organic materials for electronics comes from their lack of stability, and therefore their lifespan. Take the example of a solar cell: exposure to the sun and heat will modify the size and nanostructure of the organic photovoltaic materials, and thus degrade them. They must therefore be encapsulated to protect the devices, and this is true for all applications,” explains Lionel Hirsch, a researcher at the IMS,2 and director of the CNRS Organic Electronics research network.
Encapsulation, which is necessary for all applications, is a brake on this much-awaited flexibility. “Today OLEDs in screens are embedded between plates of glass,” explains Georges Hadziioannou, formerly of IBM and now researcher at the (LCPO). “Glass is a good encapsulant, but is by no means flexible. Our research therefore aims to both make active materials more stable and improve flexible encapsulation.
Roll-to-roll manufacturing for flexible products
Highly productive roll-processing is considered a core technology for accelerating the commercialization of wearable computers using flexible LSI. A research team led by Professor Keon Jae Lee from the Korea Advanced Institute of Science and Technology (KAIST) and by Dr. Jae-Hyun Kim from the Korea Institute of Machinery and Materials (KIMM) has jointly developed a continuous roll-processing technology that transfers and packages flexible large-scale integrated circuits (LSI), the key element in constructing the computer’s brain such as CPU, on plastics to realize flexible electronics. This roll-based flexible LSI technology can be potentially utilized to produce flexible application processors (AP), high-density memories, and high-speed communication devices for mass manufacture.
A research team led by Professor Keon Jae Lee from the Korea Advanced Institute of Science and Technology (KAIST) and by Dr. Jae-Hyun Kim from the Korea Institute of Machinery and Materials (KIMM) has jointly developed a continuous roll-processing technology that transfers and packages flexible large-scale integrated circuits (LSI), the key element in constructing the computer’s brain such as CPU, on plastics to realize flexible electronics.
Highly productive roll-processing is considered a core technology for accelerating the commercialization of wearable computers using flexible LSI. However, realizing it has been a difficult challenge not only from the roll-based manufacturing perspective but also for creating roll-based packaging for the interconnection of flexible LSI with flexible displays, batteries, and other peripheral devices.
To overcome these challenges, the research team started fabricating NAND flash memories on a silicon wafer using conventional semiconductor processes, and then removed a sacrificial wafer leaving a top hundreds-nanometer-thick circuit layer. Next, they simultaneously transferred and interconnected the ultrathin device on a flexible substrate through the continuous roll-packaging technology using anisotropic conductive film (ACF). The final silicon-based flexible NAND memory successfully demonstrated stable memory operations and interconnections even under severe bending conditions. This roll-based flexible LSI technology can be potentially utilized to produce flexible application processors (AP), high-density memories, and high-speed communication devices for mass manufacture.
Intelligent electronics to become durable, flexible and functional through new technology
With the roll-to-roll overmoulding manufacturing process developed by VTT Technical Research Centre of Finland, components can be easily overmoulded into durable electronics products such as wearable sports solutions, toys and, for instance, household appliances equipped with an overmoulded solar cell.
“VTT’s roll-to-roll overmoulding manufacturing process combined the printing, component assembly and overmoulding of components. In practice, this means that conductors, circuit boards and sensors, for instance, are printed onto a film, with the resulting electronic components then assembled by an assembly machine. Finally, the structure is overmoulded with plastic. A printable and modelled transistor was also developed with the roll-to-roll printing process during the project.