Optical sensor having heating element to heat amorphous semiconductor film

In an optical sensor, a heating element is formed on a substrate, and an amorphous semiconductor film is formed on an insulating layer covering the heating element, and is electrically insulated from the heating element. A common electrode and a plurality of electrodes are also formed on the substrate and are extended along the amorphous semiconductor film, to form cells for converting light into electrical signals, in the amorphous semiconductor film. An electric current is supplied to the heating element, to heat the amorphous semiconductor film after the film has been illuminated and photoelectric current has been picked up from the electrodes.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical sensor which can be used as, 
for example, an image sensor, and more particularly to an optical sensor 
provided with an amorphous film, functioning as a photoconductive film. 
Amorphous semiconductor film, a typical example of which is amorphous 
silicon film, can be deposited on a substrate by means of glow discharge 
of a gas, such as SiH.sub.4, or a combination of SiH.sub.4 with H.sub.2, 
PH.sub.3, B.sub.2 H.sub.6 and/or CH.sub.4. Further, it can be formed on a 
large surface area of the substrate. Due to these advantageous features, 
amorphous semiconductor film has attracted much attention in the art, 
since it can be used as a photoelectric conversion film in image sensors. 
One of the various known optical sensors using amorphous semiconductor 
films has an electrode placed in ohmic contact with the amorphous 
semiconductor film. When the film is illuminated, it undergoes 
photoconduction, and its resistance changes. As a result, the film 
generates a photo-current, which is supplied as a signal from the 
electrode. The semiconductor amorphous film and the signal electrode form 
a photoelectric transducer element, usually called a "cell", of a 
photosensor. FIG. 1 is a graph illustrating the relationship between the 
voltage applied to the cell and the photoelectric current generated by the 
cell, as the amount of light irradiated to the cell is varied. As this 
figure shows, the greater the amount of light, the higher the resistance 
of the amorphous semiconductor film. FIG. 2 illustrates the relationship 
between the luminance on the cell surface and the photoelectric current 
generated by the cell. As is evident from FIG. 2, the photoelectric 
current increases as the luminance increases. Hence, a predetermined 
electric current can be obtained by applying an appropriate voltage to the 
cell. 
It has been ascertained that the longer an amorphous semiconductor film is 
exposed to light, the higher the resistance the film will have, and hence, 
the smaller the photoelectric current will become. This phenomenon is 
known in the as Staebler and Wronski effect. That is, the photo-current 
gradually decreases as the film is continuously illuminated, even if the 
luminance (E) remains unchanged, as can be understood from the graph of 
FIG. 3. The decrease of the photoelectric current, due to this effect, is 
prominent in proportion to the luminance (E). The amorphous semiconductor 
film has its photosensitivity sharply reduced in a relatively short time. 
Therefore, the film cannot be practically used in an optical sensor. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an optical sensor which 
can compensate for the Staebler and Wronski effect in an amorphous 
semiconductor film, and which can, therefore, remain highly photosensitive 
for a long period of time. 
According to the present invention, there is provided an optical sensor 
which comprises a substrate, a heating element formed on the substrate, an 
insulating layer formed on the heating element, and an amorphous 
semiconductor film (a photoelectric conversion film) formed on the 
insulating layer. An electric current is supplied to the heating element, 
at a predetermined interval, upon lapse of predetermined time after light 
has been applied to the amorphous semiconductor film, or every time the 
photoelectric current, generated by the light applied to the film, 
decreases below a predetermined value. Whenever the heating element is 
supplied with an electric current, it heats the amorphous semiconductor 
film, whereby the film deteriorated by the application of light, regains 
its photoelectric conversion characteristic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 4A and 4B show a one-dimensional image sensor according to one 
embodiment of this invention. This image sensor comprises insulative 
substrate 1 made of glass or ceramic, or comprised of a ceramic plate and 
a glass layer formed on one surface of the ceramic plate. The sensor 
further comprises strip-like heating element 2 formed on substrate 1, 
insulating layer 4 covering heating element 2, and amorphous semiconductor 
film 5 formed on insulating layer 4 and hence, electrically insulated from 
heating element 2. Heating element 2 is either a thick resistive member or 
a thin resistive member of TaSiO, BaRuO.sub.3, Poli-Si, Cr-SiO.sub.2, 
Ni-Cr, Ti, W, or Cr, and is electrically connected at one end to 
currentsupplying electrode 3A and at the other end to currentsupplying 
electrode 3B. Insulating layer 4 is made of Ta.sub.2 O.sub.5, SiO.sub.2, 
or Si. Amorphous semiconductor film 5 is made of hydrogenated amorphous 
silicon (a-Si:H) containing 20% or more of silicon and 10% or more of 
hydrogen. It is formed by glow-discharging SiH.sub.4, or a combination of 
SiH.sub.4 with H.sub.2, PH.sub.3, B.sub.2 H.sub.6 and/or CH.sub.4 gases, 
thereby depositing amorphous silicon on insulating layer 4. Common 
electrode 8 is formed on substrate 1, in the form of a comb. Teeth 8A of 
interdigitated electrode 8 are juxtaposed, each extending in the width 
direction of amorphous silicon film 5. A plurality of electrodes 7 are 
formed on substrate 1. Each of electrodes 7 is also shaped into an 
interdigitated form and has two teeth 7A. Each of teeth 7A and 8A, and 
amorphous silicon film 5 form a cell for converting light into an 
electrical signal. Electrodes 7 and common electrode 8 are made of a metal 
such as aluminum, titanium, or manganese. A dopant such as di-boran can be 
doped into those portions of amorphous silicon film 5 which contact teeth 
7A and 8A, thereby to form n.sup.+ layers 9. When n.sup.+ layers 9 are 
formed in amorphous silicon film 5, the linearity of the voltage-current 
characteristic (FIG. 2) can be improved. To read an image, electrodes 7 
are selectively scanned, and each cell generates a photoelectric current. 
As has been explained with reference to FIG. 3, the longer an amorphous 
semiconductor film is illuminated, the more its photoelectric conversion 
characteristic will be deteriorated, and the less photoelectric current it 
will generate. However, the deteriorated amorphous semiconductor film can 
regain its photoelectric conversion characteristic when it is heated. The 
time the film needs to regain its photoelectric conversion characteristic 
depends on the temperature to which it has been heated, as is illustrated 
in the graph of FIG. 5. More specifically, as is shown in FIG. 5, the 
higher the temperature, the shorter the time the film requires to regain 
its photoelectric conversion characteristic. 
The present invention makes full use of the abovementioned nature of an 
amorphous semiconductor film. As is shown in FIG. 4A, heating element 2 is 
formed on substrate 1, in order to heat amorphous semiconductor film 5 
formed on insulating layer 4, which in turn is formed on heating element 
2. Heating element 2 is a heating resistive member of the type used in a 
thermal printing head. Its temperature therefore quickly changes, as is 
indicated in FIG. 6; it can rise quickly to 150.degree. C. or more, a 
temperature high enough to make amorphous semiconductor film 5 regain its 
photoelectric conversion characteristic. An electric current is supplied 
to heating element 2, via electrodes 3A and 3B, after film 5 has been 
illuminated and a photo-current has been supplied from each cell. Element 
2 thus generates heat, thereby heating amorphous semiconductor film 5. 
Hence, the deteriorated photoelectric conversion characteristic of film 5 
can be quickly compensated for every time the cells supply the 
photoelectric current. This ensures highly reliable image-sensing. 
Amorphous semiconductor film 5 need not be heated as frequently as this; it 
can instead be heated at regular intervals. Alternatively, an electric 
current can be supplied to heating element 2 only when the output 
photoelectric current of a reference cell, other than those cells for 
sensing an image, decreases below a predetermined value. Any of the above 
methods of heating film 5 can compensate for the deteriorated 
photoelectric conversion characteristic of film 5, without sacrificing the 
image-sensing speed of the image sensor. 
The present invention is not limited to the embodiment described above. For 
example, as is shown in FIG. 7, heating element 2 can be formed on the 
surface of insulative substrate 1, opposite to amorphous semiconductor 
film 5. In this case, substrate 1 functions as a layer which electrically 
insulates element 2 from film 5. Hence, insulating layer 4 (FIG. 4) can be 
dispensed with, thus simplifying the structure of the image sensor. 
FIGS. 8A and 8B show an optical sensor according to another embodiment of 
the invention. The sensor shown in FIGS. 8A and 8B is provided with a 
plurality of sensing areas 10.sub.1 to 10.sub.n, each of which has the 
same structure and arrangement as that of the sensor shown in FIGS. 4A and 
4B. In the sensor shown in FIGS. 8A and 8B, one heating element 2 is 
formed on substrate 1, and extends under sensing areas 10.sub.l to 
10.sub.n, and a plurality of adhesion-contact patterns 12 made of a 
high-resistivity material, the same as that of heating element 2, are also 
formed on substrate 1. Element 2 and patterns 12 are preferably made of 
Ti, W, or Cr, and formed at the same time, in the same process. Line 
electrodes 14 of Au are formed on contact patterns 12, respectively, and 
are covered by insulating layer 4 having a plurality of through-holes 16 
through which line electrodes 14 are electrically connected to 
corresponding electrodes 7. An electrode cannot be reliably adhered to 
substrate 1 made of glass or ceramic, but Au electrode can be reliably 
adhered to a Ti, W, or Cr layer. Accordingly, adhesion-contact patterns 12 
are provided on substrate 1. 
In the sensor shown in FIGS. 8A and 8B, one of common electrodes 8 and one 
of electrodes 7 are continuously selected, and a voltage is applied to the 
selected electrodes 7, 8 from a driver circuit (not shown) via the 
corresponding pattern 12 when a photo-current, that is, a photo-signal, is 
picked up from the predetermined sensor cell which is defined by the 
selected electrodes 7, 8 and a region of film 5 therebetween. An electric 
current is supplied to heating element 2 via electrodes 3A and 3B, after 
film 5 has been illuminated and the photo-current has been supplied from 
each cell. 
In the sensor shown in FIGS. 8A and 8B, the sensor can be easily 
manufactured, even when heating element 2 is formed on substrate 1, 
because heating element 2 is made of the same material as that of contact 
patterns 12 and heating element 2, and contact patterns 12 are formed at 
the same time. 
According to the present invention, an optical sensor having high 
reliability can be realized in a simple structure and into a compact 
device. Therefore, the optical sensor of the invention can be preferably 
incorporated in various systems, for example, a facsimile system. 
Moreover, various alterations and modifications can be made without 
departing from the scope and spirit of this invention.