Ultrathin Flexible Ceramics for Electronics Applications
Flexible ceramics offer desired properties such as thinness, light weight and robustness to withstand extreme conditions.
High-volume product designers traditionally considered ceramics a last substrate option due to high cost and lack of format availability. In the past, available thin ceramics were certainly not strong, robust, self-supporting or flexible. Volume markets today require a new set of high-performance, high-temperature-capable ceramic materials that are available in an ultrathin flexible roll format. These ceramics offer desired properties such as thinness, light weight and robustness to withstand extreme conditions either in operation or during the processing of component manufacturing.
In 2010, ENrG Inc. commercialized 40-µm-thick, flexible zirconia-based ceramic membranes. Thin for a ceramic, these membranes are gas tight, fully dense, strong, translucent, thermal shock tolerant and chemically inert. ENrG licensed this technology that is based on thin, sintered partially stabilized zirconia ceramic sheets developed by Corning Inc. and used in its past superconductor and solid oxide fuel cell (SOFC) research programs. The 3 mole% yttria-stabilized zirconia (3YSZ) at ≤ 40 µm thick has sufficient strength for processing and handling. The ability to relieve stress through flexing provides thermomechanical shock tolerance.
Beneficial Properties
YSZ is historically used in SOFCs and solid oxide electrolyzer cells, dental crowns, oxygen sensors, barrier coatings in harsh environments, and structural ceramics such as hip replacements. Three years after the market introduction of Thin E-Strate®, it was apparent that the membrane’s inherent properties (other than ionic conductivity) are compatible with many other market applications in harsh environments. In addition to thinness, flexibility and strength, these alternative applications tap into properties of translucency, low thermal mass, chemical resistance, infrared optical transmission, the ability to bend around very tight radii, smooth surface, dielectric property, lack of impurities, high-temperature capabilities, and strength and flexibility for roll-to-roll (R2R) handling. Applications include sensors, medical electronics, micro-batteries, solar photovoltaics (PVs), LED substrates and OLED flexible electronics.
Thin E-Strate is available in either 20- or 40-µm-thick sheets, with a possible maximum substrate size up to 120 x 150 mm. Many shapes and sizes are possible, including a standard 100-mm diameter “wafer” with flat. At 40-µm-thick, the membrane can safely meet a bending diameter of 0.5 in., and the 20-µm-thick ceramic can obtain a bending diameter of 0.375 in. (about the diameter of a pencil). An in-line, coatable format of the Thin E-Strate product will soon be available at 20-µm thick, targeted at 10-m long and with initial widths up to 3 cm. This format is being developed for those capable of coating in roll form. Additional development will scale the format to rolls of 100-300 m long and pursue wider formats.
As mentioned previously, ceramics were often not considered for high-volume applications in the past, primarily due to relative high prices (compared to the other available materials) and the lack of a roll format. The ultrathin ceramic membrane is now available via a batch process. The ability to scale to a roll format will offer a previously unavailable format for ceramics. It is expected that the translucent nature of the ceramic will aid in alignment for double-sided printing. The ceramic, unlike polymer substrates, will not stretch or deform due to humidity or the processing heat of applying coatings, or during R2R movement.
A flexible continuous “ribbon” of the membrane would allow feeds into several commercial direct write/aerosol jetting systems for products requiring unique circuit definition, such as RFIDs or other serialized/labels for harsh environments. A R2R ceramic product would enable cladding the ceramic with high-performance copper foils on R2R processing lines, further penetrating the flexible electronics circuit market.
Expanding Opportunities
The thin ceramic membrane has many qualities that make it attractive across multiple high-volume markets that require an economical roll format for R2R deposition of active and passive coatings. The future ability to scale to a R2R format offers a substrate option over stainless steel foils, thin silicon wafers, glass sheets and thick ceramic sheets.
The membrane has also proven to be compatible with many existing metal coating processes. Offering this thin ceramic in a R2R format will be a disruptive technology and a game-changer for many applications in wearable electronics, defense and aerospace, consumer products, embedded sensors, and medical electronics, ultimately moving beyond the limitations of polyimide flexible electronics.
For more information, contact the author at (716) 873-2939 or jolenick@enrg-inc.com, or visit www.enrg-inc.com.
Acknowledgements
The author would like to gratefully acknowledge funding through the New York State Energy Research and Development Authority (NYSERDA) and FlexTech Alliance. The author also acknowledges Corning Inc. for the technology license.
Exploring Additional Markets
In order to address additional market opportunities, ENrG had to demonstrate capabilities such as:
- Produce larger membrane sheets (something over 300 cm2)
- Metallize the ceramic
- Cleanly laser cut for shapes and for through vias
For metallization of the ceramic, ENrG worked initially with Oak-Mitsui using adhesive-laminated copper foils, along with typical PCB imaging and etch back processing to form various circuits. The work was successful, but it became apparent that with such a thin ceramic membrane, thinner metallization was more appropriate for coating. ENrG moved onto thin-film coatings and worked with Noel Technologies, as well as Averatek on thin film copper. The Averatek work is still progressing and has shown promise in the metallization of very fine circuits.
CEA-LITEN demonstrated CIGs solar cells and OLEDs on the membrane. Moisture degradation is a significant issue for OLEDs, and the membrane has been tested by the National Renewable Energy Laboratory. After 5,000 hours of electrical calcium testing (with test conditions of 45°C/85% relative humidity), the water vapor transmission rate (WVTR) was measured to be 1.5 ± .9 x 10-6 g/m2/day, low enough to be considered a moisture barrier.
The University of Houston assembled blue LEDs to the membrane and tested the thermal dissipation and efficiency of the LED. The LED on the membrane operated 9°C cooler than the LED on the same thickness polymide substrate.
A development program is ongoing with ITN Energy for the application of thin-film solid-state batteries on the membrane. ITN Energy successfully processed and tested a multi-up array of solid-state batteries on a large sheet of flexible 20-µm ceramic membrane. Test results were state-of-the-art for the configuration and materials set.
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