Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2001-03-02
2003-11-04
Tran, Thien (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S258000, C257S291000, C257S444000
Reexamination Certificate
active
06642541
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to an image sensor for converting an incident electromagnetic wave such as a light beam or an X-ray into electric charge and outputting an image signal by sequentially reading out the electric charge, and also relates to a method of manufacturing such image sensor.
BACKGROUND OF THE INVENTION
A known active matrix substrate for use in a liquid crystal display device, etc., includes a plurality of independently driven pixel electrodes arranged in a matrix form, and switching elements such as TFTs (Thin Film Transistors), etc., provided for respective pixel electrodes. In the liquid crystal display device adopting such active matrix substrate, an image is displayed by sequentially selecting the switching elements by scanning lines and reading potentials of signal lines into the pixel electrodes via the switching elements.
The foregoing active matrix substrate can be used for an image sensor. Examples of known image sensors adopting the active matrix substrate include: an image sensor including a conversion layer formed on an upper layer of the active matrix substrate, for directly converting incident electromagnetic wave such as a light beam, an X-ray, etc., into electric charge, wherein the electric charge generated from the conversion layer is stored in pixel capacitance at high voltage, and the electric charge is read out sequentially from the pixel capacitance. For example, Japanese Unexamined Patent Publication No. 212458/1992 (Tokukaihei 4-212458) published on Aug. 4, 1992, discloses an image sensor of the above type wherein electric charge as generated by the conversion layer is stored in auxiliary capacitance, and data (potential data) are stored in respective pixels in the form of electric charge according to the characteristics of an object. As in the case of the aforementioned liquid crystal display device, by sequentially scanning the scanning lines, for example, the data stored in a pixel selected by a scanning line is read out and transmitted via a switching element to a signal line, and an image projected to the image sensor is read out from a circuit such as an operation amplifier provided on the other end of the signal line.
The active matrix substrate, which is a precursor to the sensor in the foregoing example can be manufactured at low costs without requiring any additional facilities, because the manufacturing process for liquid crystal display devices can be used for the manufacturing process of image sensors only by adjusting the dimensions of the pixel capacitance and the time constants of the switching elements to be optimal for image sensors.
FIG. 6
is a cross-sectional view illustrating a schematic structure of a known example of the basic image sensor adopting an active matrix substrate. The structure illustrated in
FIG. 6
is disclosed in AM-LCD'99 “Real-time Imaging Flat Panel X-Ray Detector” by M. Ikeda, et al. As illustrated in
FIG. 6
, the active matrix substrate of this sensor is prepared by forming a switching element
51
on a transparent insulating substrate
55
, and further vapor-depositing thereon a conversion layer
66
and a metal layer
67
in this order. The switching element
51
is prepared by forming on the transparent insulating substrate
55
, a gate electrode
56
, an auxiliary capacitance electrode (not shown), a gate insulating film
57
, a semiconductor layer
58
, an n
+
-Si layer
59
to be patterned into a drain electrode, a metal layer
60
and a transparent electrically conductive film
61
to be patterned into a source signal line, and a protective film
62
in this order, thereby forming a substrate of the image sensor. The conversion layer
66
is provided for converting an X-ray into electric charge. The metal layer
67
is patterned into an electrode for use in applying a voltage to the conversion layer
66
. In the foregoing structure, the transparent electrically conductive film
61
is patterned into the pixel electrodes for storing the electric charge as converted in the conversion layer
66
.
In the image sensor, the electric charge is read out from respective pixel electrodes in contrast to the liquid crystal display device in which electric charge is applied to the pixel electrodes. Therefore, if a normal readout operation of a predetermined cycle is not performed due to any failure, or a trouble in signal readout program, unexpectedly large electric charge may be stored in the pixel electrode, and the resulting high voltage may cause a damage on the active matrix substrate. The foregoing problem is discussed in “Characteristics of dual-gate thin film transistors for applications in digital radiology” (NRC'96) in “Can. I. Phys. (Suppl) 74 published in 1996, in which the following structure has been proposed as a solution to the problem. That is, a pixel electrode is extended over a switching element, so that the pixel electrode can be functioned as one of the gate electrodes of a dual-gate transistor, and at or above a predetermined threshold voltage, the transistor is switched on, and excessive electric charge is released.
The structure of an image sensor which is particularly effective in preventing the foregoing problem will be explained in reference to FIG.
7
. As illustrated in
FIG. 7
, the image sensor has a so-called “mushroom structure” wherein pixel electrodes
72
and source lines
71
are formed in different layers so as to be insulated by an insulating layer
73
formed in-between, so that the entire channel region W of a transistor
74
is covered with the corresponding pixel electrode
72
. In
FIG. 7
, the reference numerals
75
,
76
,
77
,
78
and
79
indicate a gate electrode, a drain electrode, an auxiliary capacitance, a conversion layer and a semiconductor layer respectively.
The foregoing structure of Waechter, et al, illustrated in
FIG. 7
is effective for the high voltage protection in the pixel electrodes
72
. As to the size of the pixel electrodes
72
, however, significant improvement from the aforementioned active matrix substrate illustrated in
FIG. 6
cannot be expected. It is generally known that the larger is the area occupied by the pixel electrodes
72
, the more efficiently, the electric charge generated from the conversion layer
78
can be collected in the pixel electrodes
72
. In the generally used active matrix substrate, however, there is a limit for an increase in size of each pixel electrode as pixel electrodes are arranged in a plane with certain intervals from source bus lines.
In the foregoing structure of
FIG. 7
wherein the insulating film
73
is formed between the source line
71
and the pixel electrodes
72
, the pixel electrodes
72
can be formed over the source lines
71
while maintaining the insulation between them. In this state, the electrostatic capacitance is generated between the pixel electrodes
72
and the source lines
71
, and an overall capacitance of the source lines
71
when seen from the side of the signal readout circuit increases, and a noise of the readout signal is increased, resulting in lower signal to noise (S/N) ratio. For the foregoing reasons, the structure of
FIG. 7
would not offer any significant improvement in size of the pixel electrodes
72
from the conventional active matrix substrate.
In the X-ray image sensor, generally a large pixel capacitance is ensured. For this reason, the capacitance between the pixel electrode
72
and the source line
71
becomes a load capacitance to the source line
71
directly. On the other hand, internal noise generated in the signal readout amplifier is amplified by a gain in proportion to the ratio of the capacitance of the source line
71
to the feedback capacitance. It is therefore effective to reduce the capacitance of the source line
71
for a reduction in internal noise.
Further, an increase in capacitance of the source line
71
may cause variations in potential of the source line
71
corresponding to the capacitance CsD (per pixel) between the pixel electrode
72
and the source line
71
Izumi Yoshihiro
Nagata Hisashi
Sharp Kabushikikaisha
Tran Thien
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