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Amorphous Silicon Contact Imager for Office and Graphic Applications

Objective

The three main objectives for the project, containing both short and medium-term goals, were to:
-prototype a very compact contact imager with amorphous silicon sensor elements, creating a linear scanning array with better opto-electronic properties than those currently available
-investigate alternative deposition techniques for amorphous silicon (homo-cvd, photo-cvd), aiming at an increase in the stability of the deposited films
-study the integration of thin-film switches and shift registers on the same substrate, in order to avoid cumbersome and expensive hybrid interconnections.
2 scanners (10 cm long, 4 pixels per mm and 21 to 25 cm long, 8 pixels per mm) were developed, using glow discharge deposition for the amorphous silicon sensor elements. Electrical measurements on single sensor elements proved the concept. According to the readout results 2 different phenomena were apparent, each addressing different application fields: a fast (2 milliseconds per line) linear readout, and a slow but cheap matrix readout without crossovers. The sensor arrays were assembled in an A4 package, and a readout technique based on crystalline driver chips in a linear integrated mode was proposed and developed. A microfilm scanner was set up as an in systemevaluation tool. The measurements on single photosensors were compared with measurements on commercially available contact imagers. The alternative deposition techniques (homo-chemical vapour deposition (CVD) and photodissociation with ultraviolet light and argon flouride laser) were thoroughly investigated and optimized. For homo-CVD it was found that very good boron doped window layers could be produced for the sensor elements, and that the Staebler-Wronski effect played a much less severe role than on glow discharge deposited films. The most cost effective way of fabricating the complete contact imager appeared to be to integrate everything, including switches and shift registers, on the same substrate. A theoretical study proved that in this case polysilicon thin film transistors (TFT) were necessary because of speed (carrier mobility) considerations. Discrete polysilicon TFTs were fabricated using a technological process never exceeding 630 C and not using ion implantation. This resulted in a channel mobility of 16.5 square centimetres per volt second and a current on/off ratio of more than 105. Finally, the possibilities of continuous wave and pulsed laser recrystallization of amorphous silicon to produce large grain, high quality polysilicon at low temperatures were investigated.
Two scanners (10 cm long, 4 pixels per mm and 21-25 cm long, 8 pixels per mm) were developed, using glow-discharge deposition for the amorphous silicon sensor elements. Electrical measurements on single sensor elements proved the concept. According to the read-out results two different phenomena were apparent, each addressing different application fields: a fast (2 ms/line) linear read-out, and a slow but cheap matrix read-out without crossovers. The sensor arrays were assembled in an A4 package, and a re ad-out technique based on crystalline driver chips in a linear integrated mode was proposed and developed. A microfilm scanner was set up as an in-system evaluation tool. The measurements on single photosensors were compared with measurements on commercially available contact imagers.
The alternative deposition techniques (homo-cvd and photodissociation with UV light and ArF laser) were thoroughly investigated and optimised. For homo-cvd it was found that very good boron-doped window layers could be produced for the sensor elements, and that the Staebler-Wronski effect played a much less severe role than on glow discharge deposited films.
The most cost-effective way of fabricating the complete contact imager appeared to be to integrate everything, including switches and shift registers, on the same substrate. A theoretical study proved that in this case polysilicon thin-film transistors were necessary because of speed (carrier mobility) considerations. Discrete polysilicon TFTs were fabricated using a technological process never exceeding 630C and not using ion implantation. This resulted in a channel mobility of 16.5 cm2/Vs and a current on/off ratio of more than 105. Finally, the possibilities of CW and pulsed laser recrystallisation of amorphous silicon to produce large-grain, high quality polysilicon at low temperatures were investigated.

Coordinator

IMEC VZW
Address
Kapeldreef
3030 Heverlee
Belgium

Participants (3)

AGFA GEVAERT AG
Belgium
Address
Mechelsesteenweg
2520 Edegem
Centre National de la Recherche Scientifique (CNRS)
France
Address
23 Rue Du Loess
67037 Strasbourg
MBB-MESSERSCHMITT BOLKOW BLOHM GMBH
Germany
Address
Hermann Oberthstraße
8011 Putzbrunn