Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1998-10-07
2002-10-01
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S448000, C257S758000
Reexamination Certificate
active
06459132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an image sensing device typified by a photosensing device (photodetecting device) in one-dimensional and two-dimensional image reading devices utilized in facsimile machines, digital copying machines, scanners, and so on, and to a production process thereof and, more particularly, the invention relates to an image sensing device well suited for the application for wavelength-converting a radiation such as X-ray or &ggr;-ray to a photosensitive wavelength region such as visible light by a fluorescent screen and reading the converted light, and to a production process thereof.
2. Related Background Art
Heretofore, as a reading system used for input of an image information in the facsimile machines, digital copying machines, radiation sensing apparatuses or the like, there has been used an optical system using an image reducing optical system and a CCD sensor. However, in recent years, the development of photoelectric converting semiconductor material typified by amorphous silicon (hereinafter abbreviated as a-Si film) has forwarded the development of a contact type sensor in which a photoelectric conversion element is formed on a large-area substrate and in which an image information is read with an optical system having 1:1 magnification to an information source, which contact type sensor is being put to practical use.
Since the a-Si film can be used not only as photoelectric converting material but also as semiconductor material for a switching TFT, it possesses an advantage that semiconductor layers of the photoelectric conversion element and semiconductor layers of the switching TFT can simultaneously be formed.
As a typical example of a photosensor using this a-Si film, there is included a pin type photosensor such as illustrated in a schematic sectional view of FIG.
1
. Reference numeral
101
designates a glass substrate,
102
a lower electrode,
103
a p-type semiconductor layer (hereinafter abbreviated as a p-layer),
104
an intrinsic semiconductor layer (hereinafter abbreviated as an i-layer),
105
an n-type semiconductor layer (hereinafter abbreviated as an n-layer),
106
a transparent electrode, and
107
incident light. The photosensor of
FIG. 1
has the structure in which the layers are stacked in the above-mentioned order on the glass substrate
101
.
This photosensor can be operated by use of a circuit configuration such as illustrated in
FIG. 2
, in which reference numeral
110
denotes a pin type sensor,
111
a power source, and
112
a detector such as a current amplifier. In the photosensor
110
, the side C represents the side of the transparent electrode
106
while the side A the side of the lower electrode
102
. The voltage of the power source
111
is set such that a positive voltage is applied to the side C relative to the side A of the photosensor
110
.
The basic operation of this pin type photosensor will be outlined referring to FIG.
1
and FIG.
2
. When the light
107
is incident in the direction indicated by the arrow as illustrated in
FIG. 1
, the incident light undergoes photoelectric conversion in the i-layer
104
to generate electrons and holes. Since the power source
111
applies an electric field to the i-layer
104
, the electrons move toward the side C, i.e., through the n-layer
105
into the transparent electrode
106
, while the holes move toward the side A, i.e., to the lower electrode
102
. This means that a photocurrent flows in the photosensor
110
.
Further, when there is no incidence of light
107
, neither electrons nor holes are generated in the i-layer
104
. For the holes in the transparent electrode
106
, the n-layer
105
works as a hole injection inhibiting layer; for the electrons in the lower electrode
102
, the p-layer
103
works as an electron injection inhibiting layer. As a consequence, neither electrons nor holes can move, so that no electric current flows. As described, the electric current to flow in the circuit varies depending upon presence or absence of incidence of light. The operation as a photosensor is achieved by detecting the current by the detector
112
of FIG.
2
.
However, it is not easy for the above pin type photosensor to realize a photosensing device of a high S/N ratio and a low cost for the following reasons. The first reason is that the pin type photosensor has to include the injection inhibiting layers of the p-layer and n-layer. This is because in the pin type photosensor of
FIG. 1
the n-layer
105
as the injection inhibiting layer needs to have characteristics to guide electrons to the transparent electrode
106
and to inhibit holes from entering the i-layer
104
. If either one of the characteristics is missed, the photocurrent will decrease or the current without incidence of light (hereinafter referred to as dark current) will appear and increase, which will cause lowering in the S/N ratio. In order to improve the characteristics, it is normally necessary to optimize the film quality of the i-layer
104
and the n-layer
105
, i.e., to optimize film-forming conditions, particularly, various conditions including thermal treatment conditions after production of the layers.
On the other hand, the p-layer
103
, although having the reverse relation between electrons and holes, also needs to have characteristics to guide holes to the lower electrode
102
and to inhibit electrons from entering the i-layer
104
and, just as in the case of the n-layer
105
described above, it is also necessary to optimize the respective conditions for the i-layer
104
and p-layer
103
. In other words, in general, there is a difference in conditions for the optimization of the n-layer and for the optimization of the p-layer and it is difficult to satisfy the conditions of the two optimizations simultaneously. Namely, there is a possibility that the necessity for the injection inhibiting layers of p-layer and n-layer in the same photosensor may be an obstacle to formation of a photosensor with a high S/N ratio.
The second reason will be described referring to FIG.
3
.
FIG. 3
schematically shows a switching TFT. This TFT is utilized as a part of a control section in formation of a photosensing device. In the figure, reference numeral
101
designates a glass substrate,
102
a lower electrode,
107
an insulating film,
104
an i-layer,
105
an n-layer (or n
+
-layer), and
160
an upper electrode.
This switching TFT is produced in the following sequence. The lower electrode
102
functioning as a gate electrode G, the gate insulating film
107
, the i-layer
104
, the n-layer
105
, and the upper electrode
160
functioning as source-drain electrodes (hereinafter abbreviated as S-D) are successively formed on the glass substrate
101
, the upper electrode
160
is etched to form the source-drain electrodes, and thereafter the n-layer
105
is partly removed to form a channel portion
170
.
Since the switching TFT has such a property as to be sensitive to the interface state between the gate insulating film
107
and the i-layer
104
, it is normally preferable that the production process described above be carried out in the form of continuous film formation without breaking vacuum.
When the pin type photosensor described above is formed on the same substrate as this switching TFT is, the aforementioned layer structure would cause increase of production cost and lowering in the characteristics. The reason is the difference in the layer structure sequence between them; the photosensor illustrated in
FIG. 1
has the structure of the electrode, p-layer, i-layer, n-layer, and electrode in this order from the substrate side, whereas the switching TFT has the structure of the electrode, insulating layer, i-layer, n-layer, and electrode in this order from the substrate side.
This means that the photosensor and the switching TFT cannot be produced simultaneously by the same process. In other words, they are produced by complex processes in which many film-formation and photolithography step
Canon Kabushiki Kaisha
Fitzpatrrick, Cella, Harper & Scinto
Prenty Mark V.
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