Flexible and hybrid electronics

 

Flexible displays, lighting and solar panels may open the way to new kinds of consumer products and architectural features, but the real driver of the current up swell of interest in flexible, printed or hybrid electronics technology is manufacturers seeking ways to reduce production costs of large-area devices.

 

The total opportunity for electronics products that could potentially be made with low cost, low temperature processes is about $2 billion currently, reports IDTechEx, while the actual sales of printed electronics products will come to about $680 million in 2010, flexible electronics products about $360 million.

OLED displays, already the first serious organic electronics product and a $900M market, look poised for growth, as Samsung pushed the technology aggressively at the recent Society for Information Display symposium in Seattle. "OLEDs will be the next big thing in displays," asserted keynote speaker Sang Soo Kim, EVP of Samsung Mobile Display, projecting at least 600 million units shipped in five years, as the faster, brighter, lighter displays take significant share of the premium smart phone and television markets, in part because the technology allows better 3D TV.

 

 

For the next several years Samsung plans to continue to make its OLEDs with vacuum deposition on glass. It's now ramping its G4.5 OLED fab, and says the process technology is ready for the new $2.2billion G5.5 fab for 2012.

 

But beyond that, Kim said new printing processes for some steps would be needed to reduce costs. He looked to a hybrid process flow using direct printing, thin film encapsulation, and better high performance low-temperature polysilicon processes on G8 substrates to bring down costs. "OLEDs on G8 substrates will be the mainstream technology for premium TVs by 2015," said Kim in his talk. "Our $2.2B investment is just the starting point. We will lead and make our vision become reality, because we have learned that the market is created, not forecasted."

 

The company is also developing OLEDs on flexible substrates for mobile displays, and showed a video of one display that kept right on running its full color video while being bent and pounded with a hammer.

 

Suppliers say they're making good progress towards practical lifetimes for these printed patterns of organic emitters. DuPont Displays' Bill Feehery, global business director for OLEDs, reported the company is demonstrating 30,000 hour lifetimes for its printed OLED layers, sprayed with nozzles in patterns on G4 substrates with a process developed with Dainippon Screen. These printed layers are combined with cathode made by a vacuum evaporation process.

 

 

Though the commonly cited goal is a 50,000 hour life for a TV, DuPont notes in a press release that 30,000 is 15 years of realistic usage at 8 hours a day, with a 30% duty cycle. Feehrey point out these results are real printed materials, not just lab samples. He says tact time on G4 substrates is < 3minutes for cell phone quality, <90 seconds for TV patterning, and yields are comparable to other OLED deposition processes.

 

"Shadow masks can work now to G5, but for larger substrates you need a solution process," he argues. "The infrastructure is at last about ready. We can hear the train coming now," he says. "Samsung's just the one who's talking about it."

While these display makers looks at adding some printed steps to traditional vacuum deposition on rigid substrates, the alternative hybrid approach is vacuum deposition on flexible substrates in high speed roll-to-roll processes, or combined with R2R.

 

There's a lot of work in roll-to-roll manufacturing that is moving fast, and that will bring down costs, says Harry Zervos, technology analyst at IDTechEx. "It seems like almost every academic institution, research institute, and corporate research group is now researching printed logic and memory," he notes. "Nothing much has been sold yet, but that will change dramatically in a few years". The sweet spot seems to be metal oxides like ZnO, which offer better transistor performance than organic materials, but cost less than silicon.

 

 

Though roll-to-roll processes are progressing towards higher deposition rates and higher quality films, one major remaining issue remains the encapsulation technology, to effectively protect the devices from water and oxygen for a reasonable lifetime. Most effective encapsulation still requires vacuum processing, or uses rigid sheets of glass. "The first solution may be a hybrid one, with an R2R deposition process first, and then a batch process for vacuum encapsulation," notes Zervos.

 

Lifetimes for flexible electronics are already sufficient for some applications. The German company Neuber is selling solar shoulder bags and backpacks that can recharge electronic gadgets, using an integrated sheet of flexible solar cells from Konarka Technologies Inc. The company is selling 5000 units a month currently, at ~$150 each, and expects to reach volume of 100,000 units this year, for some $15 million in revenues.

Technology for vacuum roll-to-roll deposition is progressing as well. CVD processes present the most significant challenges, where it is critical to control the incorporation of unwanted dopant species from the out-gassing of the polymeric substrate during deposition and the influence of both thermal and layer stress on the planarity of the substrate, which is typically 20-120µm thick.

 

 

Applied Materials reports that its linear source technology for R2R PECVD of silicon thin films for solar cells and transistor applications is enabling film properties matching solar layers deposited on rigid substrates in a cluster tool, while achieving the throughput of batch processing. The linear design avoids the need for a showerhead, reportedly cuts process gas cost, and eliminates mid-roll NF3-based plasma cleaning. Instead the cleaning process is performed only at the end of each roll. Depending upon layer stack structure and roll length, this may occur after as long as 48 hours. Novel particle handling strategies, such as running the process vertically, help prevent particles from being incorporated in the deposited film.

 

Touch panel makers are now using Applied's R2R tool platform for PVD of optical and electrical layer stacks in a single pass at up to 6 meters per minute. For flexible PV applications, additional care has to be taken to ensure effective gas separation between adjoining processes such as reactive TCO-based back contact and metallic back reflector deposition to prevent target poisoning and degradation of material properties.

 

Patterning technology for R2R has also made significant progress over the last few years, with HP's self-aligned imprint lithography (SAIL) allowing accurate inline fabrication of transistor structures on thin flexible substrates. This technology employs a single multi-height resist pattern prior to etching successively through the various thicknesses and is capable of producing 40 nm features on 50 um thick polyimide substrates with the throughput needed for HP's R2R pilot production line.

 

 

Key issues remain with the infrastructure, however, particularly with respect to substrate materials, handling and metrology. Substrate quality is crucial and complex to quantify, both for planarized metal or polymer foils. Because the manufacturing technology is still at an early stage, yield also remains difficult to predict, with the influence, critical size and density of process-based defects still to be determined.

 

"As a relatively young industry, we're still developing the standards that will allow us to predict yield. For example, there is no standard for measuring substrate roughness", explains Neil Morrison, R&D Team Leader for PECVD Coating Products at Applied Materials' Web Coating Business Unit in Germany. "In addition to agreeing on standards for substrate properties and defect methodologies, developing the metrology tools to detect and classify defects must also be an area of focus."

 

An alternative approach to either traditional vacuum processes or printing for depositing the organic materials used in OLEDs and other flexible electronics is organic vapor phase deposition, developed by Aixtron and Universal Display Corp. from technology invented at Princeton University. The tool uses a carrier gas to deliver small molecules efficiently through a showerhead at low temperature, and at low pressure, but not high vacuum. Aixtron claims utilization rates of up to 60% of the costly organic materials, for thin films of 1% uniformity on G2/G3.5 substrates, at throughputs suitable for production.

 

 

More on the progress towards integrating these flexible and printed technologies into practical electronics manufacturing will be discussed at the session on flexible and hybrid electronics at SEMICON West in San Francisco, July 15.

 

IDTechEx will discuss the market outlook, Applied Materials and Aixtron will update on developments in process technology, Kovio will talk about its printed semiconductors, PARC its development of flexible sensors, and the Flexible Display Center at ASU its progress in hybrid process technology for displays.