Eonl ences

vacuum fluorescent display with 16 grey levels was described by Futaba Corporation (paper 16.6).

For the CRT specialist there was much to think about in the papers, in discussion and on display. Sony introduced their new 45 inch glass noster in which stress problems have been overcome without excessive weight (paper 20.1), and at the other end of the scale they demonstrated a 5 inch colour CRT for automotive applications, this being a beam index tube of startling brightness (paper 20.2). There was a paper on com- puter simulation of electron beams from the David Sarnoff Research Center (8.2), and useful paperson re- flection yokes from Vidcocolour and Matsushita (8.3 and 8.4). However, there were no reports of new phos- phors or research work in this field: have we reached the limit to the state of the art, or just a pause?

Although the CRT continues to keep pace with most new require- ments, flat-panel competitors are succeeding in their respective fields. The LCD can now be seen as the heir apparent to the CRT, having grown from infancy to adolescence in 15 years.

The demise of the CRT has been forecast for the last twenty years. Perhaps in the next twenty years it may actually happen, but it would seem nevertheless that there will be several decades yet of useful CRT life.

Conclusions High-complexity colour LCDs are available in active matrix form as

5 inch diagonal commercial products. The trend is to larger and more

complex displays with 1 024 x 1 024 pixel colour LCDs already in pro- duction on a limited scale, and dis- plays up to 12 inch diagonal being developed using a variety of LCD technologies (active matrix, double- layer STN, FELCD). Moves to larger (15-50 inch diagonal) active matrix LCDs are already being planned and active matrix LCDs are being ruggedized for military and avionics use.

Colour LCDs now offer an alter- native to CRTs for some consumer, industrial and military uses, and may be expected to reduce in cost and increase in size and complexity dur- ing the next two to three years.

Developments in polysilicon AM LCDs may allow the integration of the driver circuits into the display, but the need for this may have to be overwhelming in order to displace a-Si LCDs, given the massive invest- ment in these and their less stringent manufacturing requirements.

Electroluminescent displays are increasing in size, complexity and colour capability, but colour satur- ation and brightness still require im- provement. This could also be said of plasma displays. CRTs are mov- ing to even larger sizes and separa- tely to higher luminance at smaller sizes.

The a-Si AM LCD has become the flat panel, heir apparent to the CRT (although it may yet be dis- placed by cheaper LCDs such as FELCDs or STN LCDs, or by cheaper overall display systems using polysilicon TFT LCDs with inte-

grated drivers). This key enabling technology is the almost sole mono- poly of the Japanese corporations who, by their investment, are in an almost unassailable position when it comes to controlling the use of these displays in a wide range of new or improved products or systems. This powerful position points to their monopoly also of these, with their higher added value, and spells stra- tegic disaster for the Western elec- tronics industry unless, like Philips, they set aside short-term returns for long-term strategic growth. This seems well nigh impossible, however, given the short-term motivations of their investors and their vulnerability to take-over if their share price falls as a result.

Acknowledgements The author is indebted to George White of the GEC-Marconic Research Centre, Great Baddow, to Piero Migliorato of the GEC I-Iirst Research Centre, Wembley, and to Gordon Bonye of Rank Brimar Ltd, Sidcup, for their personal views on the highlights, for them of the sym- posium. While these have been used as additional inputs to this article, the views expressed are the personal views of the author and may not co- incide with any for all of the views of the above contributors.

lan A Shanks Thorn EMI,

Central Research Laboratories, Hayes,

UK

LCDs for Television University of Nottingham, UK 18-19 July 1988 This meeting was the second residen- tial symposium to be held by the UK and Ireland Chapter of SID. It was held in the University of Notting- ham, and gathered together an audience of 73 with a wide range of interests. It was particularly good to have participants from Europe and even a contribution from Japan.

The meeting was preceded by the

fourth Annual General Meeting of the Chapter.

Harry Ellis of Philips Compo- nents, as programme organizer, wel- comed the participants and introducted the theme of the meet- ing: a number of market analyses now predict that there will be large sales of LCD displays into the televi- sion market in the next few years. Potential markets, both in consumer

and industrial areas are measured in billions of dollars per year. The aim of the meeting was to look closely at some of the potential LCD tech- nologies and examine their prospects for television application.

Prof. Inn Shanks of Thorn EMI Central Research Laboratories pre- sented highlights of the SID Sympo- sium meeting held at Anaheim in May 1988 (see pp 105-108 for his

10~ DISPLAYS, APRIL 1989

Conferences

detailed report). A paper entitled 'The application

of amorphous silicon TFTs to LCTV' was given by Martin Poweli of Philips Research Laboratories, Redhill. The use of active matrix addressing was introduced with a brief history of the development of TFT display devices. Clearly, the demonstration by Sanyo of the first colour prototype TV in 1983 had given a strong impetus to the work of Philips. The principles of design of an amorphous silicon display sys- tem were then discussed. The major technical tasks are in being able to achieve low leakage of the TFT with good linearity of transfer from signal to LC drive, and with minimal cur- rent-crowding effects. The effect of the parasitic gate-drain capacitance was also shown to have strong effects on the drive waveform for the LC, a subject picked up later in the symposium by Powell's cotleague Alan Knapp.

Optimum TFT characteristics were shown to be dependant on the structure used. The TFT may be co- planar or staggered, and may be inverted. Philips had studied both versions of the staggered geometry, and by assessing the relative sensiti- vity to trapping in insulators, photo- sensitivity and the effects of adding passivation layers, had concluded that the inverted staggered geometry was the optimum compromise. This was borne out by the demonstration later in the day of a 6 inch prototype LCTV. Resolution was 468 (156 × 3) × 288 pixels, with impressive col- our picture quality and response characteristics. The maximum con- trast ratio is 60:1, with a value of over 20:1 for a wide range of viewing angles.

The aim of Philips is to establish a capability in the 3-6 inch diagonal range of products. Obtaining an economic yield is crucial, and the choice of TFT structure is import- ant, together with developing redun- dant or fault-tolerant structures. Significant cost reductions could also be made if some of the address- ing circuits outside the display matrix could be implemented by TFT devices. Recent reports of

mobilities in amorphous silicon of 5 cm2/Vs were encouraging, and the more traditional view that polysili- con or CdSe TFTs would be needed for addressing circuitry is now being challenged.

In the discussion that followed the paper, the question of the cost of TFT dsplay systems was raised. It was pointed out that the current price of 5 inch modules from Japan was about $800. Martin Powell's view was that the prices would not decrease as fast as some forecasts, but that costs will steadily reduce over 5 to 10 years as manufacture becomes established.

The following paper, entitled 'Present capabilities of ferroelectric liquid-crystal displays and future prospects for television', was given by the author of this review, and thus what follows is unfortunately based on what the author thought and said, hopefully not too far removed from reality. The paper represented the work at STC Technology on ferroelectric LC displays. This work is being carried out in collaboration with Hull University Chemistry Department, BDH Ltd, RSRE Mal- vern and Thorn EMI CRL. A brief introduction to the electro-optic effect was given, explaining that the main interest in FLC is the fast switching speeds, measured generally in tens of microseconds at room tem- peratures. Bistable switching devices can be made with high contrast (over 1000:1 if directly driven) and wide viewing angles. The disadvantages were that thin-cell spacings are required, typically in the range 1.5 to 2 ttm for a hail-wavelength retar- dation device.

Much progress has been made in recent years in developing FLC materials with the appropriate phase sequences. The speed at which they can be driven under multiplex condi- tions has been reduced from just over one millisecond per line to just a few microseconds per line, in the period of three years. In most other liquid-crystal devices, the visual per- formance degrades as the resolution increases. However, multiplex wave- forms for addressing FLC displays have been found so that, after

addressing, all subsequent signals do not degrade the stored optical states. In principle, therefore, displays of very high resolution can be made, without the necessity of active matrix circuitry at each pixel. Visual perfor- mance is very good with contrast ratios greater than 20:1 over wide viewing angles.

STC Technology is currently fabri- cating prototype 720 x 400 pixel displays with 300 mm diagonal, addressed at 64 ttm per line (PAL standard). Thorn EMI are also deve- loping colour and grey-scale demon- strators.

The remainder of the paper dis- cussed the design of FLC display sys- tems for a wide range of applications. A computer program had been written to evaluate the effects of all the known fabrication parameters, to determine which application specifications could be met by FLC technology.

Some limits to the technology are fairly obvious, such as those imposed by photolithography. Others are more complex, and the temperature sensitivity of the FLC speed presents particular difficulties. The FLC material itself has limited inherent operating temperature range under fixed driving conditions, but this can be increased by variation of drive voltage and addressing rate. The range available depends on the required update rate of the display and the conductor technology, which sets a lower limit for pulses addressing the display. The effect of peak cur- rent and slew rate of drivers was de- scribed and the power dissipation requirements were noted. In prac- tice, there is considerable interaction between all the limiting factors, hence the computer program. The final example shown was the result of simulating the requirements for television. Colour can be introduced using internal microdot screens, and grey-scale is achieved by multiple addressing of the display, to vary the duty cycle of black to white of the pixels. The requirements of the FLC switching speed to achieve adequate grey-scale and colour, about 14 ~ts, is within reach of recent experimen- tal materials at room temperature.

DISPLAYS, APRIL 1989 109

[onrerences

The limit due to rise time of wave- forms sets the upper size for tele- vision at about 100 cm. If photolithography gaps can be reduced to 5 pan, then the lower size limit is about 30 cm.

In conclusion, the (tantalizing?) prediction was that TV requirements may indeed be met by ferroelectric liquid crystals, although much work is still required to establish the tech- nology.

The third overview of possible LCD technologies for "IV was given by Anthony Lowe of the GEC Hirst Research Centre in a talk entitled 'Polysilicon active matrix video dis- plays'. GEC had chosen to concen- trate on polysilicon TFT active matrix displays, because of the pro- raise that eventually it would be pos- sible to fabricate the access circuitry on the same substrate and so reduce volume and costs.

An important matrix design enhancement has been devised at GEC: removing one set of access lines to the electrode facing the TFT electrode had removed the crossover short-circuit problem. This is nor- mally a major yield hazard, and also as a result pixel faults do not then so easily cause line faults, because the TFTs are capacitively coupled.

The target for TFTs using poly- silicon is to be able to use low-cost borosilicate substrates. The team at GEC have devised a process to keep the maximum deposition tempera- tures down to 600°C. They have found that low leakage can be achieved by deposition at low pres- sures (approximately 200 mTorr). In collaboration with the University of Thessalonika, they found that the lower pressure leads to large crystal grains with no microtwinning. Sub- sequent hydrogenation also helps to increase field-effect mobilities, and reduce threshold voltages.

Demonstration devices, 89 nun square and having a 256 x 256 pixel matrix, have been made and pictures of these were shown.

Work is continuing, looking at the design of shift registers using poly- silicon TFTs, at reducing the off cur- rents further and finding an alternative to ion implantation.

Anthony Lowe concluded his paper with a re-iteration of the motives for using this semiconductor material: the high process temperature leads to better stability of characteristics, it has inherent low photo- conductivity, and the high mobility also means it is less sensitive to parasitic capacitance effects.

Frances Saunders of RSRE gave the next paper on grey-scale effects in LCDs. The problem of achieving high-quality grey-scale is now becoming important as liquid-crystal TVs must at least match CRT per- formance if they are going to be accepted by potential users.

A number of options are available. Purely temporal grey-scale, where the liquid crystal has a variable duty cycle of off-to-on periods during the frame time, is only an option for fast response effects such as the ferroelec- tric. effects. Spatial grey-scale, in which the pixels are split into inde- pendently-controlled subareas, is usable if the increase in resolution is allowable and photolithography to fine dimensions can be done. The more difficult problem discussed by Frances Saunders was trying to achieve good control of grey-scale in analogue-responding pixels, particu- larly for direct-drive twisted nematic displays built over active matrices.

Best results are obtained using cell thicknesses corresponding to the first minimum in the Gooch and Tarry transmission curve, because this is where the best contrast can be obtained with the minimum of col- our variation with change of viewing angle.

Three approaches are tried to obtain shallow electro-optic transfer curves (for easy control):

• matching the natural twist to the thickness and boundary condi- tions;

• increasing the ratio of bend to display elastic constant;

• reducing the twist of the device.

Experimental results were presented for each of these cases. The first option has the effect of shifting the threshold voltage of the electro-optic effect but there is little overall ira-

provement. The ratio of elastic con- stants cannot be varied widely, but choosing two extreme mixtures does not give much difference. More effect was found by reducing the twist angle from 90"-60", but still with only a 35% increase in the width of the transition region. A penalty to be paid, however, is an increase in colour variation with viewing posi- tion.

Frances Saunders' overall con- clusion was that large improvements could not be expected in grey-scale response, and that the sigual control requirements for TFT active matrices remain with tighter toler- ances than might be liked.

Matthew Bone of STC Techno- logy gave the next paper on struc- tural studies in ferroelectric liquid crystals. Much is still to be learnt about the exact structure of the tilt and twist of the average molecular direction taken by the molecules (the 'director') in typical devices and this paper set out to describe the know- ledge about the results of one align- ment technique.

Like other groups, STC Techno- logy use cooling through nematic and smectic A phases to establish the alignment of the director in the smectic C ferroelectric phase. In thin cells, the full expected tilt angle of the director away from the rubbing direction is not seen, and this is thought to be due to the influence of the rubbing direction on the direc- tor. Apparent tilt angles are as low as one-third that expected when no voltage is applied. This effect has been studied by optical and dielectric measurements. Conoscopic exami- nation of samples show that there is no net tilt of the director, so the smectic layers are thought to be sub- stantially perpendicular to the elec- trodes. Dielectric measurements show that there is substantial tilt of the director from the surface, indi- caring that there is some accommo- dation of the conflict between the rubbing direction and the layer direction by significant distortion of the director. Further evidence comes from studies of the break-up of the structures under extreme driving conditions, when large areas can

110 DISPLAYS, APRIL 1989

[on erences

reduce the overall distortion by re- aligning the layer orientation to allow the director to line up with rubbing direction.

The difference between the driven and the relaxed structures was also demonstrated by the difference in response times between square wave drive and short pulses. In the former case, the structure approximates to the ideal case, but the latter measures the response of the relaxed state. The effects follow the predictions of the model of Xue et al., which examines the interaction of the director with the electric field as the director moves round the smectic C cone. The relaxed states are noticeably fas- ter because the director is already some way around the smectic C cone. The addition of a small AC bias to the pulse measurement allows intermediate response speeds to be obtained as the device structure is modulated between the two extreme cases .

The next paper was given by David Grant of the GEC Hirst Research Centre on fast shutters for application in field sequential video. It seems a popular thought among LCD engineers that the mere addi- tion of the simplest LCD device can transform the 'poor old-fashioned CRT' into an excellent rugged colour display, or a 3D imaging system. As the paper pointed out, the route to

~achieving the ideal combination is not simple.

The main difficulty to overcome is the speed of switching required from the shutter (less than about 3 ms, but ideally down to 1 ms or less), and the limited range of temperature over which this performance can be achieved. However, a number of commercial displays arc now in pro- duction based on such a combina- tion. Some early-design types arc based on the cholesteric-nematic 'phase-change' effect, but generally have poor extinction of the unwanted images. Two-frequency addressed nematics can be driven fast in both directions. Operating at lower power but with similar speed, the Pi-cell device from Tektronix is now in a number of commercial pro- ducts, and is capable of 2 ms switch-

ing. A bi-product of the liquid- crystal structure used in this device is a wide angle of view, but there are still limitations due to nonuni- form wavelength dependency of transmission.

Recent refinements have been the use of combinations of cells in opti- cal series; because each cell is thin- ner, its response time is less. Devices with switching times of 300 ~ts are now under development.

Tektronix have also reported that the use of cells in series, where the retardations are subtracted from each other, can achieve response times down to 100 tts by operating on the faster 'on' cell response time to determine the critical timings. The slower 'off response times are hidden by occurring simultaneously. The two cells are engineered to have iden- flcal response characteristics and so no optical effect occurs during this transition.

Much improvement can be obtained by judicious splitting of the electrodes and switching the individ- ual areas at different phases of the raster scan on the CRT. This also has the advantage of reducing power consumption, and helps to mask some ghosting effects due to unwanted persistence of phosphors.

Longer term requirements, par- ticularly for full-colour systems with high resolution and low flicker con- tent, are likely to need ferroelectric liquid-crystal shutters, which also have the advantage of wide viewing angles.

For all these systems, a potential drawback is the low transmission of the filter system. In theory, a three- colour shutter can transmit 16% of the CRT brightness, but commercial devices achieve only about 6%. However, it is not all bad news; the shutter acts as a low transmission faceplate, and actually enhances the performance in high ambient illumi- nation.

David Grant concluded his talk with photographs of some working systems. During the symposium, he also demonstrated some high-quality 3D image systems using switching spectacle shutters.

Sally Day of Thorn EMI Central

Research Laboratories gave the next talk on the modelling of two-frequen- cy addressed supertwist nematic liq- uid-crystal displays. The ambitious aim of this work was to see if the use of two-frequency materials would lead to increased speed of operation, and allow control of the transmission-voltage characteristics. Another target was to see if some of the scattering textures which cause problems in low-tilt alignment cells with 270" twist could be eliminated.

The twisted nematic device com- munity has been well served for some time by programs originally written by Dwight Berreman of Bell Labs. These solve the elastic energy equa- tions for nematic liquid crystals and enable the calculation of static and dynamic structures, and the resul- tant optical transmission. The pro- grams were modified to be able to model two-frequency materials. Using the new model, it was shown that the high-frequency drive com- ponent should indeed control the hysteresis behaviour and the slope of the electro-optic transfer curve, and this has subsequently been con- firmed by experiment.

Unfortunately, two-frequency nematics are not blessed with fast response times, and the supertwist structure also limits speed. The BDH mixture TX2A is fairly typical, with room temperature response times of the order of one second under 50:1 multiplex duty cycle using just the high-frequency drive. Addition of the low-frequency bias does reduce the switching speed by a factor of four, but this only brings it back to the speeds achieved by single-fre- quency nematics.

Sally Day also showed that it is possible to use low-flit alignments for a full 270" twist angle supertwist device. This arrangement usually generates unwanted scattering tex- tures, as was the case in the experi- mental two-frequency devices made. Application of the high-frequency drive removes the scattering texture, as suggested by the modelling stu- dies.

The overall conclusion was that the two-frequency effect was poten- tially useful in supertwist devices,

DISPLAYS, APRIL 1989 I 11

Eonl'erences

but that the inherent problems of the materials would have to be overcome before they would be used in prac- tice. These are the slow response, the temperature dependence of cross- over frequency, and the power dissi- pation of the high-frequency drive.

The next contribution came from Alan Knapp of Philips Research Laboratories, Redhill, on drive sys- tems for active matrix LCTV dis- plays. A detailed account of how to get from the RGB video input, to the picture output was given, and this complemented the earlier talk by Martin Powell. Although row (gate) drive is fairly simple, the column drivers have to sample the incoming video signals at high speed. To allow the full line address time of 64 ~ts for charging the pixel capacitance, the signal has to be sampled onto an intermediate capacitor. Tight tolerances on the switches capacitors and drivers are necessary.

There are some options on pro- cessing the incoming video signal to enhance the display image. Alan Knapp described first how this has been done to eliminate a potentially troublesome 25 Hz flicker effect from the Philips LCFV demonstra- tor. Flicker results if the TFT leaks some of the average video sig- nal on to the pixels. To ensure AC drive, the video signal has to be invented, but if this is only done at the end of each field there are differ- ent drive waveforms for pixels at the top of the display compared with those at the bottom. Inverting the video signal at the end of each line solves the problem by averaging out the leaked signal.

Gamma correction is another re- quirement for the best picture qua- lity. The LC electro-optic transfer curve is fortunately similar to that of the CRT, but shows a saturation at bright levels. This is generally not corrected at present, but perhaps will need to be for future generation de- vices.

The handling of interlaced signals also presents some interesting prob- lems, mainly when images move slowly. At present, most TFT dis- plays only show half-resolution im- ages and the problem does not arise. Full-resolution devices are expected to show break-up of the edges of moving images. The solution is to combine data from adjacent pairs of lines, at the expense of some loss of effective resolution.

The final talk of the symposium was given by Mr Takahashi from the Materials and Devices Laboratory of Mitsubishi in Japan. He spoke on the optimum design for LCTV using amorphous silicon TFTs, covering not only TFT structures, but also the colour matrix arrangement and the choice of liquid-crystal configur- ation.

The dimensions used for the TFT are based on a compromise between the 'o~' currents, parasitic capaci- tance effects and the active area available, within the limits imposed by fabrication at high yield. The general trend was to use shorter gate lengths. Reliable insulator f i l l s were now achieved by using double layers of materials, and the yield was now high enough to allow the incorpo- ration of storage capacitors at each pixel, to help control of the drive voltage.

Mitsubishi now use projection photostepping for their lithography. They have found it superior to con- tact masking in resolution, alignment accuracy and edge quality, and the masks can be used indefinitely because there is no damage due to contact with substrates. Their TFT structure is an inverted staggered type with an extra light shield, capable of six orders of magnitude modulation of the channel current.

A detailed analysis was given of the choice of the colour pixel arrangement and the shape of the subpixels. The final choice was made on the basis of achieving the highest

effective resolution, with minimum unwanted artefacts, and turned out to be a rectangular array, with two green, one red and one blue pixel. As other groups are opting for differ- ent arrangements, it will be interest- ing to see the differences between the final products.

Mr Takahashi then turned to the subject of the design of the liquid- crystal structure. Most devices are made which are normally black under no drive (i.e. they use crossed polarizers). It is possible to use a normally white mode, but there must be enough voltage to get a black state at full drive. This has the advantge of slightly wider viewing angles and less unwanted colouration. Best per- formance is also achieved with cell retardation tuned to the first mini- mum of the Gooch and Tarry trans- mission curve.

The talk was concluded with pic- tures of one of the latest develop- ments from Mitsubishi which is 25 mm diagonal and has 640 × 240 pixels. High contrast, 42:1 at normal incidence, is achieved with a nor- mally black mode configuration. The contrast is above 10:1 for over 80 ° of arc in both horizontal and vertical directions.

Chapter Chairman Neil Bartlett closed the symposium with thanks to the speakers.

A straw poll of attendees sug- gested that most had found it a use- ful meeting, not only for the organized event but for the opport- nity to meet old and new colleagues and discuss the ways of the display world. The efficient arrangements provided by the University of Nott- ingham, were much appreciated.

Peter Ross STC Technology Ltd,

London Road, Harlow,

UK

112 DISPLAYS, APRIL 1989