a-Si:H Image Sensors: Some Aspects of Physics and Performance
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A-SI:H
IMAGE
SENSORS:
SOME ASPECTS OF PHYSICS AND PERFORMANCE
K. ROSAN and G. BRUNST Siemens Research Laboratories,
D-8000 Minchen 83,
West Germany
ABSTRACT High photosensitivity and fast photoresponse have proved a-Si:H to be a suitable thin-film photoconductor for large-size linear image sensors. Besides the a-Si:H bulk properties, the a-Si:H/electrode interfaces are of major influence on the senthe readout performance, sor performance. In view of this circuit has to be designed carefully with respect to the desired dynamic range. Care has to be taken to avoid electrostatic hazards when mounting the sensor, as the breakdown voltage of a-Si:H sensor elements was found to be about 60 volts. INTRODUCTION Communication and office automation are more and more calling for document reading devices, such as facsimile equipment, smart copiers, and document scanners for word processing systems. In conventional equipment, charge-coupled device (CCD) image sensors are used. However, a long optical path and accurate optical adjustment are required for image size reduction, because the dimensions of these CCD sensors are much smaller than those of document sheets. Therefore, contact-type linear image sensors have been developed for compact reading Here a rod lens devices. array having only one-tenth of the conventional optics focal length, projects the document without reduction onto the sensor. In contrast to chalcogenides and CdS/Se, ease of fabrication, material stability, and fast photoresponse have proved amorphous silicon (a-Si:H) to be a suitable thin-film photoconductor for image sensors. work is The aim of this to discuss some aspects concerning the nature of the amorphous-silicon/electrode and to compare interfaces readout methods different with respect to the desired dynamic range. PREPARATION Contact-type linear image sensors can be produced
Mat. Res. Soc. Symp. Proc. Vol. 70. ' 1986 Materials Research Society
Fig.
1: Detail of a linear a-Si:H image sensor
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in a hybrid technology, using monocrystalline silicon readout chips connected to the light-sensing elements of the amorphous silicon sensor. A sample fabricated in this way is shown in Fig. 1. The photoelement array consists of a 1pm thick photoconducting film of glow-discharge amorphous silicon, sandwiched between an array of metallic back electrodes and a transparent top electrode (ITO). The resolution of this sensor is 12 elements/mm, with an element size of 75pmx75)m and a spacing of 10pm between the elements. The circuit pattern interconnecting sensor and chips is formed of gold and contains lines as small as 22pm. Patterning was done by photolithography. A glass plate was used as substrate. The readout chips were die-bonded to the glass substrate and then connected by wire bonding to the circuit pattern. An important feature concerning sensor fabrication is the breakdown voltage of the a-Si:H sensor elements, i.e the ESD sensitivity. The breakdown voltage was found to be about 60V. PHYSICAL ASPECTS To ensure constant film quality, the bulk
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