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Materials and Technologies for High-Mobility TFTs for Liquid Crystal Display-Bus Drivers

Objective


The objective of this project was to develop polycrystalline silicon thin films and technologies for column bus drivers and line bus drivers to replace external chemical vapour deposition (IC) for peripheral driver circuits of active matrix liquid crystal displays (LCD). The main task was to develop a low temperature deposition process for the thin semiconductor films compatible with the glass substrates used for the LCD. The investigated deposition processes were plasma enhanced chemical vapour deposition (CVD) and electron gun evaporation, whereas low pressure CVD was used to establish the thin film transistors (TFT) process technology. Furthermore, elementary TFT circuits, suitable for bus driver shift registers, were investigated and developed. Driver prototypes of short length were also combined with LCD matrices and tested.
technologies can also affect other applications in large-area microelectronics, such as motionless scanners.
The objective of this project was to develop polycrystalline silicon(pSi) thin films and technologies for column-bus and line-bus drivers to replace external ICs for peripheral driver circuits of active matrix liquid crystal displays(LCDs).
The main task was to develop a low-temperature deposition process for the thin semiconductor films compatible with the glass substrates used for the LC display. The investigated deposition processes were Plasma Enhanced CVD and Egun evaporation, whereasLow Pressure CVD was used to establish the TFT process technology. Furthermore, elementary TFT circuits, suitable for bus driver shift registers, were investigated and developed. Driver prototypes of short length were also combined with LCD matrices and tested.
Progress and Results
The following results were achieved:
-Polycrystalline Si films prepared below 600 oC on borosilicate glasses were processed to produce TFTs.
-TFTs, prepared from Egun Si films deposited at 550 oC, used an atmospheric pressure CVD-deposited SiO2 layer. These TFTs showed field effect mobilities up to 16cm2/V and enabled a better yield than TFTs using plasma-processed gate insulators.
-With PECVD, the capability exists to prepare Si3N4 transistors with field effect mobilities up to 35 cm2/Vs in Silayers.
Using these deposition processes there is no need to use the costly process of ion implantation to obtain TFTs with the requested characteristics. Furthermore, at the above-mentioned deposition temperatures high-quality glass substrates can be used instead of the expensive quartz substrates needed for higher temperatures. Nevertheless, additional investigation will be necessary to reduce the deposition temperature to values where cheap soda lime glass substrates can be used.
A new TFT-equivalent circuit was inserted into the SPICE simulation programme, from which a five-level inverter with static TFTs could be simulated. Tests of the dynamic behaviour of this programme were performed, and good correlation with actual TFT was found.
Although the project was originally planned to last 5years, it stopped after in May1988, after only 3 years, by common agreement between the partners, following the decision of CNET to work on a production line in a competitive environment using some ofthe initial results (two-masks process for TFT manufactured with LPCVD-deposited pSifilms).
The goal expected for the end of the project (mid-1990), ie driver circuits on soda lime glass able to address a 300x300pixels LCD matrix with a resolution of 4pixels per millimetre, has obviously been not achieved. But the work scheduled for the lasttwo years in this project is now a part of the ongoing ESPRITproject on Active Matrix LCDs (number 2283). It is included in this new project as a specific task to be carried out by the same companies, AEG and CETIA.

Coordinator

AEG Olympia AG
Address
Goldsteinstraße 235
60528 Frankfurt Am Main
Germany

Participants (3)

CNET France Télécom
France
Address
2 Route De Tregastel
22300 Lannion
Compagnie de Signaux et d'Équipements Électroniques (CSEE)
France
Address
Rue Marcelin Berthelot Zone Industrielle Toulon Est
83088 Toulon
THOMSON CETIA
France
Address
Rue Marcelin Berthelot
83078 Toulon