A Review of the Opportunities and Processes for Printed Electronics (Part 3): Materials, Process Developments
Most of the attention given to materials has appropriately centered on conductive inks, especially silver. Silver is the most conductive commonly used metal for making circuit conductors. Membrane switch circuits, which operate at relatively high voltages and low currents, have been printed onto polyester base materials using silver inks for more than a few decades. The challenge has been getting these circuits to have the bulk conductivity associated with copper. Common inks have conductivities that hover around 10% of copper and are not generally suitable for higher performance applications that operate at lower voltages or may require more power.
A number of suppliers have attacked the problem using a combination of new formulations of binders in the ink and nanoparticle silver are showing good improvements. One company, NovaCentrix, is addressing the challenge of making printable circuits by developing a tool and technology that sinters metal and semiconductor inks in a matter of milliseconds using light energy. The process reportedly can be carried out on a range of materials, including low-temperature, flexible substrates such as polyethylene terephthalate (PET) and paper. It is a promising technology for making low-cost products.
Organic and inorganic materials are both used for printed electronics. Ink materials must be available in liquid form for solution, dispersion or suspension. They must function as conductors, semiconductors, dielectrics or insulators. Material costs must be appropriate for the application.
For printing, viscosity, surface tension and solid content must be tightly controlled. Cross-layer interactions, such as wetting, adhesion, solubility and post-deposition drying procedures, affect the outcome. Additives often used in conventional printing inks are unavailable because they often defeat the purpose of electronic functionality.
Material properties largely determine the differences between printed and conventional electronics. Printable materials provide decisive advantages beside printability, such as mechanical flexibility and functional adjustment by chemical modification (e.g., light color in OLEDs).
Printed conductors offer lower conductivity and charge carrier mobility. With a few exceptions, inorganic ink materials are dispersions of metallic microparticles and nanoparticles. P-type metal-oxide-semiconductor (PMOS) technology may be used in printed electronics, but not complementary metal–oxide–semiconductor (CMOS) technology.
Conductive inks have been available for at least 40 years. The new conductive inks are designed specifically for use with low-temperature substrates including paper, (PET), polyether ether ketone (PEEK) and other plastics (including polyethylene film) and cure in an air environment.
Table 1: Ink Technologies
Copper, tin and silver nanoparticles are used with screen, flexo, offset and inkjet printing. Gold, silver and copper particles are used with inkjet. Sheet resistance is as low as ten milliohms per square. Resistivity’s as low as four times bulk have been attained with silver, but are higher for copper inks.
AC electroluminescent (EL) multicolor displays can cover many tens of square meters, or be incorporated into small watch faces and instrument displays. They involve six to eight printed inorganic layers, including a copper-doped phosphor, on a plastic film substrate.
Nanotechnology is the greatest boon to PE inks. There are proprietary material comprised of silicon nanoparticles dispersed in an environmentally-friendly blend of chemicals. These have optimized silicon particle size and dopant concentration to maximize the conversion efficiency of photovoltaic (PV) cells. The ink is screen-printed and a lower viscosity is available for inkjet printing.
Carbon nanotube (CNT) is another nanotechnology. CNT inks are available from many suppliers and known as V2V Ink Technology. C3Nano has transparent conductive CNT ink for touch panels and solar cells.
Table 2: Nanoparticle Conductive Metal Inks (from Novacentrix)
Printed electronics uses flexible substrates, which lowers production cost and allows fabrication of mechanically flexible circuits. While inkjet and screen printing typically imprint rigid substrates like glass and silicon, mass printing methods nearly exclusively use flexible foil and paper. PET is a common choice due to its low cost and higher temperature stability. Polyethylene naphthalate (PEN) is another.
PEEK is a colorless organic polymer thermoplastic used in engineering applications. Polyimide (PI) foil is another alternative. Paper's low cost and manifold applications make it an attractive substrate, but its roughness and absorbency make it problematic for electronics. Low roughness and suitable wettability, which can be tuned pre-treatment (coating, corona) are important criteria for substrates. In contrast to conventional printing, high absorbency is usually disadvantageous.
Table 3: Applications and substrates
Flexible materials are a key characteristic of PE. Many products traditionally utilize glass to protect the active layers. To replace glass, the flexible substrates need to be an effective barrier against oxygen and water vapor, be sufficiently strong not to rip or tear and, if a cover, transparent to visible light. Many plastics, such as such as Mylar, polyimide, PET and ORMOCER, have these characteristics. Substrates can even be papers and paper hybrids. Applications and substrates are summarized in Table 3.
The majority of the world's PET production is for synthetic fibers (in excess of 60%) with bottle production accounting for around 30% of global demand. In discussing textile applications, PET is generally referred to as simply polyester while PET is used most often to refer to packaging applications. The polyester industry makes up about 18% of world polymer production, followed by PE and polypropylene (PP).
PET consists of polymerized units of the monomer ethylene terephthalate, with repeating C10H8O4 units. PET is commonly recycled, and has the number 1 as its recycling symbol. PEN is a polyester with good barrier properties (even better than PET). Because it provides a good oxygen barrier, it is well-suited for bottling beverages, such as beer, that are susceptible to oxidation. It is also used in making high performance sailcloth. PPG’s Teslin is a polyolefin matrix with high silica filler.
Table 4: Characteristics of Substrates
DuPont’s Teijin Films are polyester films that are pre-treated on both surfaces to promote adhesion to most industrial coatings. Melinex, Mylar, Cronar and Teonex are registered trademarks of DuPont Teijin Films. Teslin and Teijin Films are designed for PE application like RFID, batteries, OLEDs and sensors.
PEEK is a semicrystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained at high temperatures. The Young's modulus is 3.6 GPa and its tensile strength is 90 to 100 MPa. PEEK has a glass transition temperature of around 143°C and melts at around 343°C (662°F). It is highly resistant to thermal degradation and attack by both organic and aqueous environments. At high temperatures, it is attacked by halogens and strong Bronsted and Lewis acids, as well as some halogenated compounds and aromatic hydrocarbons.
If it is very thin, glass can also be a flexible substrate. AGC and Corning have both developed ultra-thin glass (only 0.1mm thick) for OLEDs and other applications. Alkali-free glass is composed of silicon dioxide, boron oxide and aluminum oxide and is free of sodium and potassium, so it is used widely as a substrate for thin-film transistor liquid crystal display (TFT LCD) and OLED. Soda-lime glass is composed of silicon dioxide, sodium oxide and calcium oxide and is widely used in construction, automotives and many electronics devices.
Recently, a research group at the Hebrew University of Jerusalem has developed a nanotechnology ink that self-sinters at room temperature. The researchers have demonstrated that the new silver nanoparticle-based ink can be deposited and provides post-print conductive properties without a post-sintering process step. Instead, the properties of the nanomaterial are used to make the ink aggregate and self-sinter. The elimination of the temperature needs associated with normal sintering processes opens the door to processing cost reduction and the use of lower cost, temperature-limited substrates, including paper and lower melt-temperature plastics (Table 4).
Other companies are developing catalytic inks that can be electrolessly or electrolytically plated to achieve the necessary conductivity. One company of note—Conductive Inkjet Technology—has been developing a range of equipment for printing catalytic circuit patterns on material webs in a roll-to-roll fashion. When the catalytic ink is electrolessly plated, the company reports that the conductivity ranges from 50mΩ to 20mΩ per square. The company’s technology, which has an approach that is similar to a technology developed in Silicon Valley around 1990, can reliably produce typical feature sizes down to less than 250μm, which is finer than earlier technology that used a laser printer and catalytic toner. While some have performed its initial process and equipment development on an inkjet platform, the company's literature indicates that they can produce 50μm features using flexographic printing, which could prove to be an important departure.
Flexographic printing, as a follow-on technology in the same market space, opens the door to higher production rates and offers the potential to print both sides of a two-metal layer circuit in a single pass. With some creativity, it is possible that inkjet printing could be similarly adapted. One concern about this technology is that it will produce circuits that will build both vertically and laterally, reducing the space between circuits and potentially causing shorts.
Post plate-up technology shares many of the same concerns as laser printing technology in terms of managing feature dimensions in process. Metrics are usually predicated on three items:
- Resolution, or how many dots per unit length
- Drop size, or how small each dot is
- Print rate, or dots per second
These metrics combine to measure how quickly and efficiently a circuit pattern can be printed. Keep in mind that smaller dot size is not always better since it may result in decreased productivity with minimal benefits in terms of resolution.
Laser equipment suppliers have developed a method that is an outlier in the realm of printing flexible films. In simple terms, the method is the reverse of printing. A thin film of metal on a flexible base is ablated by a laser beam that prints the reverse (negative) of the circuit pattern leaving a thin metal circuit pattern. One company, LPKF Laser & Electronics AG, had demonstrations on 35mm film, which is a good deal narrower than the current wide web focus area, but it might fill a niche for special products or particular needs.
Beyond simple conductors, resistors and light-emitting diodes, there are still more important emerging technologies for printable semiconducting materials. Printed semiconductors may be the most important and distinguishing features of a printed electronic circuit. A joint venture by two European firms PolyIC GmbH & Co. KG and Thin Film Electronics ASA has produced fully functional, thin film, rewritable polymer memory products using a high-volume, roll-to-roll printing process. The developers claim that the process is high-yielding and suitable for low-end consumer products that require memory, such as toys and games.
Another company, Terepac Inc., which is tackling printed semiconductors from a different approach, has developed a transfer printing process for semiconductor devices using a simple principle of photopolymer release. The method takes advantage of the fact that certain polymers used as adhesives decompose thermally to gases over a narrow temperature range with little or no residue. Additionally, they discovered that the temperature can be drastically reduced by the action of a photogenerated catalyst. The material has been labeled a digital release adhesive (DRA) because it first has the adhesive properties of a typical thermoplastic polymer and then no adhesive properties at all after gasification, since only air is left. In production, the minuscule semiconductor components (as small as 100μm) are first transferred en masse to the printing plate in a laminator called the prepress. The plate has been coated by DRA and may be rigid, or could be produced on a flexible web. In the printing step the components attached to the carrier with the DRA are irradiated through the plate in only those regions where the components should be released, allowing great precision.
Another developer in California has shown breakthroughs in nanomaterials and printing technology. Kovio Inc. has also developed semiconducting, silicon-based inks that allow for the integration of printed silicon electronics and thin-film technology. The company suggests that the technology will enable the fabrication of stable, high-performance, low-power integrated circuits that can operate at frequencies in the MHz range and potentially above. An advantage of printed silicon technology over organic electronics alternatives is that printed silicon purports to offer significantly higher performance, lower power consumption and environmental stability. The economic viability of the process compared to processes using organic materials is not known at this time, but the solution should prove compelling for some applications, such as RFID tags.
In Part 4, Happy Holden talks about the market applications for printed electronics.
You can read the Part 1 of this article series here, and Part 2 here.
Editor's Note: This paper has been published in the proceedings of SMTA International.