By Debra Vogler
WaferNews Technical Editor
Beginning a presentation at MRS’ recent Spring Conference with a photo of rat bone tissue may seem a bit unorthodox, but Paul Calvert, professor of materials science and engineering at the U. of Arizona, did have a point. Scientists are searching for new ways of toughening ceramics and composites and the need for a material that mimics bone tissue is also needed for medical implants.
“Biological growth is a layer-by-layer process where cells work together, rather like an inkjet printer in expressing solidifiable material,” explains Calvert.
Calvert went on to present results obtained using inkjet printing – with highly modified Hewlett-Packard printers – to build organic electronic devices and/or structures. He maintains that inkjet methods can be used in many more formats than simply printing single layers of a passive ink. For multi-layer printing, the problem is ensuring that new layers of ink don’t mix with previous layers. So the issue becomes how to chemically solidify the previous layers, which Calvert suggests could be accomplished by using thermal or photo-crosslinking, or by codepositing two reactive inks. Subsequent printing can be carried out using either thermal or piezoelectric print heads. As a result of his efforts to print and immobilize dielectrics, metallic conductors, and polymer gels, Calvert reported achieving a resolution of 50-micron, which is adequate for many display applications.
A topic that comes closer to traditional semiconductor manufacturing is that of organic thin film transistors (OTFTs). Because processing of OTFTs can be done at low temperatures, they can be deposited on a wide variety of material, such as polymeric substrates, cloth, paper, etc. As Thomas Jackson, professor of electrical engineering at Penn State, put it: OTFTs enable electronics anywhere – and the performance of the best OTFTs is now rivaling, and in some cases exceeding, that of amorphous silicon devices.
Jackson also presented comparison data between two different types of OTFTs – showing that napthacene OTFTs had mobility about one order of magnitude less than pentacene OTFTs, but napthacene circuits were slightly faster (~25%). The threshold voltage and subthreshold slope of the napthacene devices offered some circuit advantage over pentacene devices. Jackson believes the pentacene devices can be modified to gain the advantages of napthacene and have substantially better performance.
Progress aside, the field of OTFTs is still in its infancy. Taking these devices to the next level – the manufacturing floor – will need additional work. An example of a relatively high-priced, high-margin product incorporating OTFTs is a flexible, polymeric substrate display. However, questions remain about device stability, reliability and reproducibility, according to Jackson.
“But the best demonstrated results are comparable to, or better than, hydrogenated amorphous silicon devices – the most commonly used transistor technology for direct view flat panel active matrix liquid crystal displays,” notes Jackson. “This needs a manufacturer to decide there is money to be made to make the investment needed to move to manufacturing.”
Low price/very low price/low margin products could include electronic bar code tags (e.g., RF ID tags), smart cards, and sensors for food quality.
“For products of this type, pieces other than the organic transistors also need substantial work,” states Jackson.
Being able to do the patterning at very low cost, as well as with improved resolution and reduced defect density, will be the major challenge.
“Another part of the problem is that many of the interesting products are low-margin commodity parts and it can be difficult to develop high-tech solutions for these since the investment recovery is often not clear. There is interest and activity from a number of players, but commercial reality is farther off – at least three years and more likely five to 10 years,” notes Jackson.
FlexICs is one company that is trying to bring manufacturing considerations to bear in the area of organic light emitting devices (OLED) display backplanes on flexible substrates. Several challenges the company faces in using plastic substrates include not exceeding a process temperature of 100 degrees C, and accounting for thermal expansion and shrinkage. According to Daniel Toet, director of process development at FlexICs, the company is also working with tool suppliers to make processing tools capable of handling flexible substrates. To overcome the technical problem of plastics not being compatible with conventional dopant activation techniques (i.e. furnace anneal and RTA), the company is using pulsed laser annealing. This method allows dopants to diffuse rapidly in melting silicon without affecting the substrates.
Another future concept covered by a number of presentations at the conference was nanowires – which could be used for such future applications as nanoscale electronic and photonic devices. While commercial applications belong to the future, the well-known vapor-liquid-solid crystal growth mechanism – used many years ago to grow whiskers – is now used by Peidong Yang, assistant professor of chemistry at UC-Berkeley, to construct nanowires. Made in this manner, nanowires are defect-free, according to Yang, unlike typical ICs with multilayer thin films.
“For our superlattice nanowire, we can get around the problem [of lattice mismatched materials] by relieving the lattice strain laterally,” comments Yang.
Practical applications of nanowires include nanolasers, nanosensors, and nanocomputers. Regarding sensor applications, Yang explains that, “Conventional oxide (polycrystalline thin film) sensors operate at 300 to 500 degrees C in order to enhance the surface adsorption/desorption kinetics. This is not desirable for sensors that operate in an explosive environment. Our sensor uses single crystalline nanowire as the active element, at room temperature, with comparable sensitivity.”