Displays smaller than a stamp

Current display-based portable computing products force users into compromising image size, resolution, colour and brightness in favour of portability and battery life. But a new generation of displays will enable wearable monitors for notebooks and hand-held PCs to become a reality.

In the UK, research into the development of a colour miniature liquid crystal flat panel display was undertaken by the SLIMDIS project, a UK Link Collaborative between GEC-Marconi Research and three other industrial partners Admit Design Systems, Davin Optical and Swindon Silicon Systems and the University of Edinburgh. Aimed at head-mounted applications, the display developed in 1997 combined conventional CMOS and ferroelectric liquid crystal technologies to achieve a degree of miniaturisation which, they claimed, was unachievable with thin-film transistor active matrix technology.

The result of the SLIMDIS work was the development of a 1024 by 768 XGA display, operating from PAL video and RGB data sources. Based on a 12 m pixel (representing greater than 2000lines/in resolution), the display was fabricated on a 16.3mm by 16.3mm silicon substrate and had a pixel array area of 12.3mm by 9.2mm.

The display was addressed using a conventional matrix address scheme. As the backplane was opaque, the display was illuminated from the front (using a polarising beam splitter) and viewed with appropriate magnifying optics. The display was sequentially illuminated using three LED arrays (red, green and blue). Such a scheme avoids the need for three separate pixels to achieve colour and cuts the price and complexity of the display as no colour filters are required.

The Displays Team from Admit Design who had developed the digital and analog microdisplay interface systems within the UK-sponsored SLIMDIS programme, and the Displays Department within CRL, based in Middlesex, have now formed Micropix with the purpose of bringing microdisplay components and applications to reality. Incorporating proprietary MicroPix ferroelectric liquid crystal over silicon, the company’s latest microdisplay has a diagonal of less than 16mm and a resolution of 1024 by 768 pixels.

BORN IN THE USA

In the US, several companies including Colorado Microdisplay (CMD), Three-Five Systems and Displaytech have also already developed and commercialised miniature display devices.

CMD’s microdisplay uses the company’s reflective LCD technology. Once believed to be too slow for field sequential colour, the application of CMD’s special nematic liquid crystal display technology has now resulted in a high-resolution, full-colour sequential colour display with a 0.47in diagonal active-matrix structure.

The display is composed of high mobility crystalline silicon CMOS-process transistors that form individual pixels. These transistors drive nematic liquid crystal material placed on top of closely packed mirrors.

Analogue pixel values are routed to cells by built-in row and column shift registers operating synchronously with the pixel clock and control signals. The analogue values create a voltage across the liquid crystal while polarised light reflects off pixel mirrors through the liquid crystal, a polariser and a viewing lens.

While other Active Matrix Liquid Crystal Displays (AMLCDs) use red, green and blue sub-pixels to create composite colours on a pixel triad, the CMD8X6P uses a field sequential colour technique to produce all three colours on one pixel.

Field sequential colour means that a full-colour image is produced in successive red, green and blue images. All three sub-images are produced three times faster than conventional CRT or TFT display images and fused together by the human visual system into a full-colour image.

The reflective microdisplay incorporates a beam splitting optical element which allows a very compact folded optics design. This creates a high resolution 800 by 600 pixel display in the same physical volume as a limited resolution 320 by 240 pixel (QVGA) transmissive display.

The company claims that field sequential colour technology also enables the microdisplay to operate on one-third the pixel requirement of a spatial RGB display. Traditional transmissive displays perform at less than 5% overall efficiency, but reflective displays from CMD produce close to 40% system efficiency.

LIGHTCASTER TECHNOLOGY

The Displaytech Lightcaster VGA display panel is a miniature reflective display that consists of a ferroelectric crystal applied to a CMOS integrated circuit. The VGA panel is built on a flexible printed circuit with a ceramic backing for mounting.

To build the device, specially synthesised ferroelectric liquid crystal (FLC) material is applied to the surface of a reflective CMOS integrated circuit, providing a fast switching, full colour, video-capable display. The device functions electrically as a write-only memory consisting of 640 by 480 bits. Each memory cell switches the layer of FLC directly above it between two optical states. One state reflects incoming polarised light with no change in polarisation, while the other rotates the plane of polarisation by 90 degrees. An external optical polariser produces light and dark pixels.

A glass window coated with a single transparent electrode covers the display. During normal operation, this electrode is held at 2.5V, while the underlying pixels switch between 0 and 5V. This produces electric fields across the FLC material of approximately +/-2.5V/micro m.

Exploiting the fast switching speed of the FLC materials, full colour is achieved by sequentially displaying the individual bitplanes for each primary colour in rapid succession. The interface to the device is a 24-bit parallel data bus that accepts binary pixel information in bitplane order.

For its part, Three-Five Systems, in an alliance with National Semiconductor, offers the LCoSTM (liquid crystal-on-silicon) microdisplay on a space much smaller than a postage stamp. This reflective silicon-on-glass technology, which operates in either projection or virtual modes, currently provides SVGA or SXGA resolution, and a contrast ratio greater than 200:1.

According to Mike Worboys of the Marconi Technology Centre, in contrast to the situation in the US, miniature displays in Japan are in their infancy. Hitachi has admitted to having a significant activity in this technology, but few details are in the public domain. At Samsung, work is underway to develop a projector based on an XGA resolution ferroelectric LCOS DRAM display from Displaytech.