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Large-Area Complex LCDs Addressed by Thin-Film Silicon Transistors

Objective

Liquid Crystal Displays (LCDs) have emerged to become the dominant flat-panel display technology in recent times. Their low power and voltage requirements are advantageous, not only for portability, but also because they lead to unstressed highly reliableelectronics which, when combined with the lack of inherent failure modes in LCDs, yield very thin lightweight displays with service lives orders of magnitude longer than Cathode Ray Tubes (CRTs). Thinfilm transistor addressing of LCDs allows an increasein display size and complexity to levels adequate for word processor and graphics displays.
The aim of this project was to investigate a viable technology for the fabrication of large area (A5toA4) and complex (up to 1000addressable lines) LCDs based on polycrystalline and amorphous silicon transistor active matrices. Silicon Thin Film Transistors (TFTs) showed higher yield and reliability than other TFT materials.
Extensive investigation of the following areas was made:
-optimisation of transistor characteristics, yield and stability over large areas
-special high throughput, large-area semiconductor processing equipment
-special LCD fabrication techniques
-colour systems
-interactive displays
-human factors.
The aim of this project was to investigate a viable technology for the fabrication of large area and complex liquid crystal displays (LCD) based on polycrystalline and amorphous silicon transistor active matrices. Silicon thin film transistors (TFT) showed higher yield and reliability than other TFT materials. Extensive investigation of the following areas was made:
optimisation of transistor characteristics, yield and stability over large areas;
special high throughput, large area semiconductor processing equipment;
special LCD fabrication techniques;
colour systems;
interactive displays;
human factors.
A breakthrough in the technology of polysilicon TFTs on glass allowed GEC to achieve, in a reproducible manner and in small-geometry (10x10micron) devices, on-off ratios in excess of 105, mobilities of 10cm2/Vs and threshold voltages as low as 8 V. Anew active matrix circuit was patented which entirely eliminates line failures and this increases the yield enormously. These breakthroughs were verified on test displays 6x4cm in size with 96x64pixels, which were fabricated with less than 10defect
s. In the amorphous silicon (aSi) area, in addition to the CNET proprietary technology, ThomsonCSF developed an alternative potentially highyield process based on smallgeometry (10x20micron) highmobility (0.4-0.7cm2/Vs) TFTs and demonstrated almo
st defect-free 6x8cm displays with 256x320pixels.
CNET demonstrated an 8x8cm colour display with 320x320pixels based on a new RGB colour filter technology. A system based on capacitive interaction with resolution of 0.4mm was realised. AEG achieved very low pinhole density (1percm2) sputtered si
licon dioxide. Modulex developed drive circuitry interconnections using high throughput tape automated bonding of chips on to flexible printed circuitboard. Significant achievements were also made in the areas of plasma silicon nitride and large area dis play photolithography.
Following the end of the planned work schedule, a number of prototype displays were demonstrated at the ESPRIT Conference in September1987. A major follow-up project, which exploits the results achieved in these studies, is now going on in ESPRITprojectnumber2283.
Exploitation
Some recent surveys have underlined the importance of new display technologies because of their potential for eventually replacing CRTs as the principal visual display medium for a range of informatics based products. The progress on topics listed above places the collaborators in a strong position, even with respect to the Japanese competition, particularly because of the availability of both polysilicon and amorphous silicon technologies.

Coordinator

GEC-Marconi Materials Technology Ltd
Address
Elstree Way
WD6 1RX Borehamwood
United Kingdom

Participants (6)

AEG Olympia AG
Germany
Address
Goldsteinstraße 235
60528 Frankfurt Am Main
ARISTOTELES UNIVERSITY OF THESSALONIKI
Greece
Address

54006 Thessaloniki
CNET France Télécom
France
Address
2 Route De Tregastel
22300 Lannion
MODULEX A/S
Denmark
Address
Kloevervej 101
7190 Billund
Thomson CSF
France
Address
Domaine De Corbeville
91404 Orsay
Università degli Studi di Bologna
Italy
Address
Viale Risorgimento 2
40136 Bologna