ZBD - the zero-power display

A ZBD Displays product story
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Edited by the Electronicstalk editorial team Jun 6, 2003

Zenithal bistable technology is a significant evolution in liquid crystal display technology.

Although similar in construction to the simple LCDs used in watches and calculators, ZBDs use a 'grating' structure to give the novel property of surface-induced zenithal bistability.

These microstructured surfaces, very similar in form to the holographic diffraction gratings found on credit cards, hold the LC molecules at the surface, either vertical to the internal surface plane (the continuous state) or parallel to it (the defect state).

The molecules are held in either of these stable orientations, one of which is black and the other white.

These two LC orientations, black and white, are both stable and can only be switched from one to the other by a voltage pulse of appropriate polarity.

This 'bistability' is not affected by thermal or mechanical effects, so that once an individual ZBD pixel is switched from 'black' to 'white', or vice versa, it remains in that state, even when the power is switched off.

ZBDs therefore only require power when an image is updated, resulting in enormous power savings.

By comparison, most conventional LCDs - such as TN (twisted nematic), STN (supertwist nematic) or active matrix - require continual application of a voltage to each pixel to maintain the desired liquid crystal orientation - even if the overall displayed image is static.

The achievement of zenithal bistability results in: an image that can be stored indefinitely without power; and a rugged image that is retained even after severe mechanical flexing and shock, and is tolerant to wide manufacturing variations such as cell gap.

Passively addressed, ZBDs can write very complex images a line at a time and hence offer higher resolution and higher information content without requiring the additional cost, or complication, of an active matrix thin film transistor (TFT) backplane.

This allows for a large amount of information to be displayed clearly and quickly and makes them a cost-effective alternative to active matrix TFT displays, such as those used in laptop computers.

ZBDs are capable of four times the contrast and 60% higher brightness than conventional STN LCDs, providing a reflectivity of at least 36%.

They also feature a wide viewing angle without the need for compensation films that are "best-in-class" for monochrome displays, making it possible to achieve TFT front-of-screen performance at STN costs for colour displays.

ZBDs achieve 120-degree horizontal and 90-degree vertical viewing angles for contrast ratios over 10:1.

ZB displays require only standard gas/water barrier layers, unlike LEPs (Light-emitting polymers).

They are compatible with plastic substrates, and can operate in transmissive, reflective or transflective modes, depending on the user environment.

ZBDs can be constructed to provide greyscale by varying the grating shape within individual pixels with no additional manufacturing costs, thereby paving the way to colour displays that rival TFT displays' front-of-screen performance at colour STN costs.

As a simple modification to the established processes found on existing STN lines, ZBDs have the advantage of being able to leverage all the technological advances made in other LCD technologies, including brightness enhancement, electronics and colour filters.

In addition, the gratings for ZBDs are readily manufactured using simple techniques such as photolithography and embossing.

As a result, ZBDs offer wide tolerances that make the manufacturing of complex displays on flexible or standard plastic substrates easier than ever before.

The technology is also easily adaptable to existing passive matrix LCD production lines, bringing the promise of a low-cost, high-volume 'roll-to-roll' manufacturing process within reach.

With that will come obvious weight savings and reduced susceptibility to breakage, along with a new dimension of design freedom for consumer electronics OEMs.

The combination of zero power between image updates with high information content and suitability for flexible or standard plastic substrates means that ZBDs are placed at the heart of future handheld consumer devices.

These characteristics offer large power savings, especially in applications with low image update rates (mobile phones, PDAs, portable GPS etc), thereby extending battery life and substantially reducing device size and weight at a competitive cost.

In addition, the ultralow power and superior readability of ZBDs are essential requirements for products such as electronic readers and web tablets.

ZBD technology provides substantial competitive advantage.

Conventional displays, for example STN and TFT and other emerging technologies, for example organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs), all suffer from one or more of the following significant drawbacks - high costs, high power requirements, limited image complexity or the expense and difficulty of developing and optimising completely new manufacturing processes.

Other bistable technologies, such as bistable twisted nematic (BTN), electrophoretic inks and cholesteric displays also have one or more significant drawbacks compared to ZBDs.

These include tight manufacturing tolerances, higher operating voltages, lower operating speeds and temperature ranges, inability to move to transflective geometries, difficulty in moving to plastic substrates, difficulty in producing a high number of colours, or lack of stability of the display under mechanical pressure.

Although ZBDs can be customised for different applications, potential applications share a number of key specifications.

These include low power consumption (less than 10V at 20ms per line for a device with a 4um cell gap), images to be written quickly (in less than 30us per line at 35V, again, for a device with a 4um cell gap) and wide temperature ranges (from -20 to more than +80C).

Potential applications and benefits include e-books and electronic readers.

Storage densities and processing speeds are now making it possible to contemplate very thin, lightweight electronic books for the purposes of education, business and leisure.

In fact, an initiative by China's Ministry of Education has been undertaken to replace printed books used by its 240 million students with electronic devices, reducing the cost of distributing educational material and eliminating the physical strain experienced by children lugging heavy backpacks to and from schools.

Unlike conventional LCD technologies like STN and other emerging technologies like OLEDs and LEPs, which require the image to be rewritten 15 to 75 times per second, whether or not the image is changing, images on a ZBD are retained permanently after the power is turned off - indefinitely.

This is an important capability in applications where the display is located remote from a power source.

These characteristics offer enormous power savings for applications such as e-books and electronic readers, where the image is updated infrequently and the display can show a personalised screen saver when the instrument is not being used.

In addition, users can enjoy months and months of reading without the need to replace or recharge the batteries.

This extended battery life also offers significant reduction in size and weight, enabling a product design freedom never available before.

Low power, unbreakable displays have long been on the roadmap of smartcard manufacturers, with many failed attempts over the past few years.

To give the required flexibility and physical ruggedness and provide long-term image retention, ideally without an onboard battery, embedded displays for smartcards such as loyalty cards, frequent flyer cards, transit cards, credit cards and phone cards must be made from plastic and offer zero power image storage.

The image on a ZBD is unaffected by mechanical pressure applied to the display or to flexing as would be the case for any display embedded in a smartcard.

This opens the door to credit and loyalty cards that will be able to display expenditures to date, accumulated mileage or points and other similar information.

Updated when 'swiped' or accessed in a 'contactless' fashion, a card would require absolutely no battery power in the interim.

Increasingly sophisticated images are required by mobile applications, including web enabled mobile phones, smartphones, pagers and personal digital assistants (PDAs), where there is a growing need to download e-mails, web pages and other high information content images.

This is driving demand for larger displays of excellent optical quality and very low power consumption.

ZBDs' dramatic battery saving capability means that designers of devices such as mobile phones and PDAs can create smaller units at less cost.

Although similarly passively addressed, a ZBD does not suffer from the inferior optics and information content limitations of a conventional STN display, allowing ZBD to compete openly with more costly active matrix TFT displays in these high growth applications.

The ZBD is resilient and gives improved contrast and brightness, storing the image even under mechanical stress.

ZBDs can readily operate in transflective mode, enabling readability in both dark conditions and bright sunlight.

This feature is impossible for bistable cholesteric, electrophoretic ink, LEP and OLED displays.

In the same respect, electronic shelf edge labels (ESELs) used for real time price updating, product information, advertising and promotional schemes, would benefit enormously from the improved optics, reduced power and indefinite image storage that a ZBD delivers.

Although ESELs have achieved some market penetration over the past few years, it has been modest and restricted by the difficulty in generating attractive returns as only limited information can be displayed today, making only a small dent in reducing costly back-store activities.

ZBDs provide a solution that will allow for complete product information to be displayed without fear of total loss of information should batteries run out, a major step in redefining in-store product management processes.

Images on ZBD displays have been stored for several years without degradation despite mechanical stress applied to the screen.

Designed with the business traveller in mind, the ability of Microsoft's Smart Personal Objects Technology (SPOT), such as the smartwatch, to go for months without needing to recharge or replace the batteries is critical to the business traveller accessing important information.

However, the more information displayed, the more power consumed.

ZBDs, by displaying sophisticated images with high visibility and readability for extended periods of time, without constantly needing to be recharged and without image degradation, provide an improved path to commercialisation for smart watch manufacturers.

In addition, ZBDs also provide the resilience and toughness essential for a device carried on the person.

Taking advantage of its wide manufacturing tolerances, ZBDs can also be made in plastic.

In time this will allow zero-power watch displays to be produced in a curved manner appropriate to the wrist.

The higher contrast, reflectivity and viewing angle of a ZBD is much more akin to the performance of a high-price TFT display than to that of a traditional low cost STN display, with the added benefits of ultralow power and image storage.

ZBD is the only passively addressed technology to rival active matrix LCDs, offering equal performance, lower power and image storage at roughly half the cost.

The combination of these factors gives ZBDs a strong competitive advantage and meets any objections associated with the technology.

With its superior front-of-screen performance and unlimited information content, coupled with ultra-low power and rugged image storage, ZBDs are the clear choice for future handheld devices.

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