Printed Electronics Edge Into Market Niches

Paula Doe, SEMI, San Jose, www.semi.org -- Semiconductor International, 6/13/2008 8:43:00 AM

Flexible printed electronics may seem like a radical departure from standard silicon microelectronics, but like most revolutionary technologies that do make it to the market, flex circuits are actually edging into first applications by making use of as much of the existing infrastructure as possible.

“We’re seeing a lot of evolutionary change and hybrid approaches,” said Kevin Cammack, director of technical marketing and industry development for the U.S. Display Consortium (San Jose), noting technologies ranging from modifying standard vapor deposition to use on new clear plastic substrates to ink jetting new organic semiconductor contacts onto fabricating traditional single-crystal silicon solar cells. Also driving some current interest is the inherent “greenness” of the printing approach, with its smaller bill of goods, less exotic materials and lower energy consumption.

“The $100B display industry is going to be the big driver, especially for hybrid technologies designed to take advantage of the existing $120B in fully depreciated manufacturing capacity to make lower cost or lower power displays,” he said, noting that most of the new technologies coming online for flat panel displays (FPDs) are inherently flexible, and progress is being made on many fronts. New developments include clear flexible substrates that can be processed at up to 350°C without softening or hazing, and new processes that allow transistor deposition at temperatures as low as 300°C.

E-books will start to get far more interesting later this year when models built on flexible plastic display backplanes start to hit the market, said Michael McCreary, vice president of R&D at E Ink (Cambridge, Mass.).

The portable readers are starting to get some traction, with Amazon recently reporting that books for the Kindle already accounted for 6% of sales of the 125,000 titles available in both traditional and Kindle formats. But e-book models available so far still attach the company’s flexible bi-stable electrophoretic imaging film to traditional thin-film transistor backplanes, deposited on glass with expensive LCD equipment.

“We’re working with 10-15 major companies on making printed and flexible backplanes,” McCreary said. “These non-traditional products will start to come out within months later this year.” He pointed to Polymer Vision’s (Eindhoven, Netherlands) rollable display that pulls out of a cell phone, which is slated to hit the market later this year, based on relatively conventional semiconductor processes and a layer transfer process. Plastic Logic (Cambridge, UK) will have reportedly printed one million sheets with its organic transistors this year. E Ink is already printing its imaging film by the kilometer with a roll-to-roll process.

1. E Ink moves pigment chips with electrical charge to create the image. (Source: E Ink)

McCreary outlined the wide variety of approaches he sees making significant progress, with different trade-offs between temperature and electron mobility, ease of manufacture and quality of transistors. LG Display (Seoul, South Korea) is using traditional lithography and tools on a flexible stainless-steel substrate that withstands higher temperatures. Epson (Long Beach, Calif.) is using traditional lithography with polysilicon for higher mobility, first making the circuits on glass, then transferring the layers to plastic, then transferring them again to flip them right-side up to get silicon mobility on plastic. Samsung (Seoul, South Korea) is opting for a simpler production process with more work on transistor quality, making low-temperature circuits directly on plastic.

One promising approach to making a low-cost backplane for displays borrows continuous thin-film deposition technology from the solar industry, then uses an imprinting process rather like flexo printing to lay down a single 3-D masking structure for etching all of the film layers, avoiding the usual alignment issues.

Hewlett Packard Labs (HP, Palo Alto, Calif.) and PowerFilm Solar (Ames, Iowa) first deposit all of the thin-film layers on the roll of flexible substrate, and then top them with a polymer layer that can flex and stretch with the substrate. The masking pattern is then impressed into the polymer by a rolling cylinder, creating a 3-D structure that encodes all of the patterning information for the film stack by a discrete height for each step. The film is etched through the mask, then one level of the mask is etched away to expose a new pattern and the next layer of the film stack etched, until the complete film stack is patterned. Improved material for the stamp now reportedly maintains precision patterning through thousands of impressions on a kilometer of substrate without the typical swelling or fracturing.

2. Imprint stamp for roll-to-roll self-aligned imprint lithography. (Source: Hewlett Packard)

Carl Taussig, director of HP’s information surfaces lab, argued that keeping the traditional stack materials and vacuum deposition processes limits the need to develop new materials for the electronics, while going to a continuous process simplifies handling and appears the best way to reduce costs, at least for large-area substrates with moderate integration demands. “We felt this kind of a system would get to a device sooner,” he said. “The materials research is always the slowest part, so the less of it we have to do, the better.” He said work is now focusing on yield and scaling issues, so development is getting to the point where it’s going to need to move out of the R&D lab to the next level.

Ramping up to volume yields on the continuous R2R process, however, presents significant challenges for metrology and inspection. “Everything has to be done inline and integrated into the process control,” Taussig noted. “To control thickness, for example, you can’t just set the machine up to deposit a 45 nm layer; you have to measure thickness inline. You have to do end-point control of etch while the process is in a steady state. For high volumes, you need a way to do optical inspection and defect repair while the substrate is still on the web. The whole process monitoring, detection and repair area is a fertile area for development,” he said, “and it’s wide open now.”