Printed posts secure large plastic LCDs

Imprinting arrays of micron-sized posts and other structures is key to the development of new, large area, low cost, full colour flat displays. Optimised to perform as addressable paper, the technology uses no driving transistors, so is currently too slow for video, although fast enough for static displays. Unlike conventional LCDs, it is non-volatile, so once the pixel cells are latched, they stay that way, so power consumption is minimal. Some of the individual parts of the technology are not particularly new, but their combination is certainly unique, and since it comes from one of the world's largest companies, Eureka expects to see A0 drawing sized displays shortly. The development comes out of HP Laboratories in Bristol, who simply refers to the technology as a 'printed plastic colour display'. Non-volatility is achieved by having arrays of 1 micron high, imprinted posts, which protrude 20% to 30% into each cell of liquid crystal. By applying suitable voltages, liquid crystal molecules can be made to either lie down, or stand up draped around a post. As there are about 50,000 posts in a 100 dpi (dots per inch) pixel, it is possible to arrange for sub-regions of the pixel to switch at different voltages. In the current prototypes, there are four sub regions, giving five levels of grey, which with three colours, gives 5x5x5 =125 possible colours. However, either by having more sub regions, or simply making the pixel cells very small and switching suitable numbers of them on and off, it should be possible to generate as many apparent colours as is desired. Switching pulses are about 0.5ms long, so a 1,000-line display should take only 0.5s to write as a passive matrix. The prototype display takes several seconds to write fully, but this, the developer said, is because of limitations imposed by the software and driver in the PC used to demonstrate it, and not the display itself. Once switched, the cells maintain their state, as exemplified by a monochrome research display written two years ago, which remains unchanged since the power was last switched off. The display structures are imprinted on thin plastic substrates that are fairly flexible. Researchers said that they have been laid down on a "range of standard plastics", including polyethylene terephthalate (PET). The imprinting process used to produce the posts, may also be used to imprint cell boundaries and templates into which colour filters may be deposited, since conventional inkjet printing is not considered to be sufficiently accurate to produce complex structures on a fine scale in repeated print passes. Imprinting cell boundaries also allows colour cells to be made with relatively small gaps between them, enhancing overall brightness, with metal conductor tracks only five microns wide deposited onto the imprinted templates. Conduction to the tops and bottoms of cells is via conducting polymer. Indium tin oxide as used in conventional LCD displays, if laid down on plastic "can be made to work", but the conductivity is not considered to be high enough for medium to large displays. Dr Adrian Geisow, manager Displays Research at HP Laboratories told Eureka, "The key is to get everything working together." The latest prototype is illuminated from behind, although Huw Robson, manager of the Digital Media Department said, "We have work in the labs working in the reflective dimension." Dr Geisow considers that the development is, "still a few years away from commercial production", but it looks as if manufacturing costs should be low. The latest prototype is only 40 x 30mm and 128 x 96 pixels, but the goal is to make A3 and A2 and possibly even A0 sized displays, which it is hoped, will be "five times cheaper" than is possible using other technologies. The present state of the technology is the result of five years of technical development. Potential uses include e-books and e-magazines. Such devices have been prototyped at HP Labs and elsewhere, but Dr Geisow considers that even at 1280 x 1024 resolution, other display technologies do not provide sufficient resolution to allow easy reading of fine print. One of the goals is to achieve resolutions similar to those achieved with the firm's commercial inkjet printers. Resolution of the prototype display is 100 PPI (pixels per inch) although the target is 200 or greater. Flexible LCD displays on the sides of motor vehicles were suggested by Eureka in the July 2000 cover feature story. These were inspired by displays developed by British Industrial Graphics in Witney, which depended on polymer dispersed liquid crystal displays (PDLCs) originally developed by Raychem. These are quite flexible but have no grey scale, and Eureka was shown prototypes developed for yacht instrument displays with photo-luminescent backlights. One of the advantages of PDLCs is that they do not require polarisers, but on the other hand, they do require direct drive or an active matrix. Hewlett Packard Company Email Dr Adrian Geisow Eureka says: It looks as if flexible, thin, low cost plastic display replacements for paper could be with us in a few years after all, perhaps replacing the paper book or newspaper, enabling A0 full drawing sized displays, and possibly covering car bodies to enhance appearances and improve safety. Readers should not expect, however, that such a development will lead to truly paperless offices. Pointers * Displays are very thin, flexible, and apart from back lights when required, consume insignificant amounts of electric current. * An imprinting manufacturing technology has been developed that has the potential to produce them at about one fifth the cost of competitive technologies. * Development is presently at the stage of being a working full colour display prototype, 40mm x 30mm, able to produce 125 colours.