TVs that hang on the wall like a portrait have for decades had a regular part in popular predictions of how we will live in ‘the future’. In real life, however, the technological problems involved in producing efficient thin, large area displays have not yet been successfully addressed.

Now that digital wide screen programming and domestic web browsing are becoming commonplace, there is suddenly a real consumer demand for high information content displays in the 50cm to 100cm diagonal size range. Contending flat panel technologies for this slice of the market, expected to be worth about US$30 billion worldwide by 2002, are struggling with a plethora of materials issues. The aim is to produce an affordable, energy efficient, light weight, thin and broad area display. Significant problems arise with scaling existing technologies, processing these complex electronic devices with as high a yield as possible, and ensuring they operate uniformly and predictably for long periods of time. Figure 1 summarises current and emerging display technologies.

Figure 1. The range of different competing display technologies depends on the size of the display.

In many ways the standard is set by the now ubiquitous cathode ray tube (CRT) which has long dominated the domestic market. The CRT is an affordable and reliable means of generating bright moving images, only let down by being as deep as it is wide. The ever wider screens now available are increasingly bulky, heavy and difficult to manufacture.
Liquid Crystal Displays – LCD’s

The most successful flat panel display (FPD) technology to emerge so far is the active matrix liquid crystal display (AMLCD) found in laptop computers, space saving desktop monitors and many other consumer electronic items. Compared to the domestic television these screens suffer from a poor viewing angle, and their brightness relies on an energy sapping colour filtered back light. Screen Technology Ltd is overcoming these drawbacks with its photoluminescent LCD (PLLCD) technology. This uses a near ultraviolet (390nm) back light modulated by an LCD shutter to excite a phosphor screen on the front of the display. It is already demonstrating 30cm colour displays and is now working on an innovative approach to seamlessly tile LCDs using PLLCD technology to enable large screens to be made.

Even so, AMLCDs are unlikely to extend affordably much beyond the 50cm diagonal in the near future. They require three thin film transistors positioned at each colour pixel (one to control each primary colour), so the display is rather like a very large area memory chip. Producing a defect free device over such a broad area, in a clean room where dust particles are almost certainly present, is a real challenge. Not only that, but as the size extends, the capital equipment costs rise very steeply. Some of the tolerances necessary for the liquid crystal to operate reproducibly are tight, requiring extremely flat glass substrates and good registry between the numerous deposition, photolithography and etching steps. For example, the metal tracks are deposited by magnetron sputtering, the transparent indium tin oxide electrodes are produced using reactive ion sputtering, and the transistors are made using plasma enhanced chemical vapour deposition at around 300°C.