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20 Nov 2002
The marketing hype of the last 10 years, namely that LCDs are replacing CRTs as the first choice in many applications for direct-view displays, seems to have been justified. Chris Williams of displays consultant Logystyx investigates.
From Displays Europe October 2002
The LCD is king. Well, maybe and maybe not. LCDs are already coming under increasing pressure in the microdisplay projection market, where the highly successful polysilicon transmissive TFT LCDs and the more recent reflective liquid-crystal-on-silicon displays are under severe commercial threat from Texas Instruments' digital light-processing (DLP) chip. DLP chips are now widely available, often at lower prices than comparable LCD solutions, with the result that DLP-based projectors are taking the larger market share in the portable and domestic projector markets.
Of course, LCDs have facilitated the creation of new market applications that could not be satisfied by any other display technology: hand-held devices, PDAs, GPS terminals, portable DVD players, notebook and laptop computers to name but a few. LCDs have a secure and profitable future in this area - or do they? Competition is coming in the form of organic LEDs (OLEDs) - and if you believe the market hype, OLEDs will displace LCDs from many of these applications.
Why is this? Natural evolution? Are LCDs doomed to extinction? Unlikely - but LCD displays do have a problem, and this is where many of today's researchers are focusing their activities.
Keeping up with Moore's law
The problem is one of speed - displays in the market today simply aren't very good. Semiconductors have mostly kept to Moore's law, which roughly translates as speed and complexity doubling every 18 months. This usually means a halving in cost along with the added performance.
Displays, on the other hand, have not kept up with Moore's law. While semiconductors raced ahead, displays have been gently meandering behind. If displays had obeyed this law, then today's laptop computers would use displays with resolutions of 5000 x 2000 pixels or more within a 15 inch diagonal. We would routinely have 10 million instead of 1-2 million pixel displays. Of course, very-high-resolution displays are just arriving on the market - IBM's Bertha display, with 9.2 million pixels, sets the standard for best market practice, and L G Philips, Sony and others are slowly introducing 2 and 3 million-plus pixel displays.
Given that manufacturers like Sharp, Sony and Samsung are already selling 30 inch diagonal LCD TVs to the high-end domestic market, and Samsung is looking to offer 40 inch diagonal LCD products soon, the race is on to create the mass market for LCD TVs to supplement the success of the LCD desktop monitor market.
Reviewing where LCD displays are at present, we see that many of their earlier shortcomings have been successfully addressed:
• Viewing angle New device construction techniques with descriptive names such as Vertically Aligned Nematics, In Plane Switching and Super Advanced Super Fine TFT from a host of manufacturers now routinely create devices with very wide viewing angles and negligible colour variation with angle. However, performance comes at a price, and many laptop and LCD desktop monitor products are still built with cheap displays rather than best-practice displays.
• Colour gamut Better colour filters to increase colour purity and saturation, and closer matching of the filters to the output of carefully selected triphosphor backlight cold-cathode fluorescent lamps (CCFLs), have extended the colour gamut of (best-practice) LCD displays to approach, match or even exceed that of many cheap CRTs. But the cheaper LCD products don't come anywhere near, using lowest-cost colour filters and off-the-shelf CCFLs.
• Luminance This is one situation where size really does matter. Consider LCD projection devices: Seiko-Epson is offering its Dream III polysilicon TFT light engine module with a luminance of 3000 lm, from a display cell measuring just 0.99 inch diagonally. This contrasts with a typical direct-view 15 inch desktop LCD monitor with a luminance of 250-300 lm. Projection cell LCD makers can use microlenses to collect the incoming light and focus it through the aperture. A secondary microlens array at the front surface of the LCD cell can then be used to disperse the light according to system requirements. Direct-view displays have more difficulty in using microlenses because of the dimensions and geometries involved.
• Switching speed Switching speed is a function of two components: the LCD cell and the active driving circuitry for TFT LCD displays. The LCD fluid and display-cell manufacturers have developed fluids and assembly constructions that can switch pixels from black to white and, more importantly, between adjacent grey scales in around 15 ms. This will probably be acceptable for many low-resolution multimedia applications, but it is still not fast enough to eliminate visual artefacts when displaying full-motion videos which need switching speeds faster than 10 ms.
For volume production of large substrates at low cost, a lower temperature production process must be developed so that conventional low-cost glass substrates can be used and factory production time reduced. The race to create low-temperature polysilicon substrates has been run for the last few years, and is likely to continue for several more. Different approaches to creating polysilicon are under development. Conventional approaches use heat treatments, but novel developments using lasers and ultraviolet treatments are under way at universities and companies around the world.
Sharp has already demonstrated devices using its newly developed continuous-grain silicon process. Here, amorphous thin films are deposited and then recrystallized using surface treatments.
All of these substrate-deposition developments would be subject to review if manufacturers look to implement plastic substrates. Cross-compatibility of production processes between glass and plastic substrates is very limited. Plastic, particularly where roll-to-roll in-line production techniques are needed, would require the reinvention of many of today's processes.
Substrate materials research is where some of the most interesting work is taking place. Corning, Schott and other glass companies, as well as various universities, are experimenting with combining glass with polymer technology to create ultrathin, lightweight substrates that offer the clarity and barrier properties of glass, but exhibit the formability and shock resistance of plastic. Research is under way to develop 100 and 50 µm thick substrates.
There is a tremendous amount of development that must take place to bring LCDs back on track with Moore's law. The next few years will probably be more exciting in terms of display breakthrough than the last two decades, as laboratory "proof of concept" demonstrators are engineered into mass production. If you can't wait for the products to hit the streets then visit the SID symposia at their various locations around the world. This is where the world's best emerging technology displays are shown to an eager audience of scientists, engineers and the press.
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