Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2001-02-28
2003-12-23
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370010
Reexamination Certificate
active
06667481
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates two-dimensional image sensors for detecting electromagnetic radiation (X-rays, visible light, infrared light, etc.) images.
BACKGROUND OF THE INVENTION
A conventional two-dimensional image sensor for electromagnetic radiation images is provided with semiconductor sensors arranged in a two-dimensional matrix form. Each semiconductor sensor is equipped with an electric switch and, as it detects X-rays, visible light, and other kinds of radiation (hereinafter, the description will focus on X-rays which represent all kinds of radiation), produces electric charge (electron-hole). The electric switches are activated one row at a time to measure the electric charge produced by the semiconductor sensor in each column.
Two-dimensional image sensors of this kind are described in terms of specific structure and detecting principles in, for example, D. L. Lee, et al.,
A New Digital Detector for Projection Radiography,
SPIE, 2432, pp.237-249,1995 (published in May 1995); L. S. Jeromin, et al.,
Application of a
-
Si Active
-
Matrix Technology in a X
-
Ray Detector Panel,
SID '97 DIGEST, page 91-94, 1997 (published in May 1997); Japanese Laid-Open Patent Application No. 6-342098/1994 (Tokukaihei 6-342098; published on Dec. 13, 1994); and other documents.
The following will discuss the arrangement and detecting principles of a conventional two-dimensional image sensor in reference to FIG.
4
and FIG.
5
.
FIG. 4
is a plan view and a cross-sectional view taken along line A—A of the plan view, both showing a conventional two-dimensional image sensor.
FIG. 5
is a cross-sectional view showing a single pixel in the two-dimensional image sensor of FIG.
4
. The cross-sectional view of
FIG. 4
does not partly show the structure in detail so much as FIG.
5
.
The two-dimensional image sensor of FIG.
4
and
FIG. 5
has a basic structure in which a photoconductive film
102
and a common electrode (upper common electrode)
103
are formed in this order on the active matrix substrate
101
.
The active matrix substrate
101
includes: a glass substrate
104
; TFTs (Thin Film Transistors)
105
as switching elements and charge storage capacitances (Cs capacitances)
106
provided on the glass substrate
104
; and pixel electrodes
107
provided so as to cover all these members. Pixels, each of which is constituted by the TFT
105
, the charge storage capacitance
106
, and the pixel electrode
107
, are arranged in an X-Y matrix form (two-dimensional matrix form).
The TFT
105
is constituted by a gate electrode
108
, a gate insulating film
109
, an a-Si (amorphous silicon) layer (i-layer)
110
, an a-Si layer (n
+
layer)
111
, a source electrode
112
, and a drain electrode
113
. The charge storage capacitance
106
is constituted by a storage capacitance electrode (Cs electrode)
114
, a gate insulating film
109
, a storage capacitance electrode
113
(which also acts as the drain electrode
113
).
The pixel electrode
107
is electrically insulated from the TFT
105
and the charge storage capacitance
106
, as well as electrode wires connected to these members, by an intervening insulating layer (insulating protection layer)
115
and an insulating layer (insulating protection layer)
116
. The pixel electrode
107
and the drain electrode
113
are electrically coupled via a contact hole formed through the insulating layers
115
and
116
.
The photoconductive film
102
is provided so as to cover the pixel electrodes
107
and the insulating layer
116
in the active matrix substrate
101
. The common electrode
103
is provided so as to cover the photoconductive film
102
.
The photoconductive film
102
, when irradiated with X-rays, produces electric charge (electron-hole) in it. The photoconductive film
102
is made from a semiconductor material, such as a-Se, that is selected depending on the wavelength of the electromagnetic radiation to be detected.
The common electrode
103
and the storage capacitance electrode
114
are arranged so that an electric voltage can be applied across them.
In the active matrix substrate
101
, under the insulating layers
115
and
116
is formed a wiring layer
120
which includes, among others, the gate (scan), source (signal), and Cs (capacitance) lines coupled respectively to the gate electrodes
108
, the source electrodes
112
, and storage capacitance electrodes
114
for the individual pixels. The cross-sectional view of
FIG. 4
shows a gate line as the wiring layer
120
.
The wires extend to edges of the glass substrate
104
which are outside a pixel region
118
(region in which the pixel electrodes
107
are disposed). The wires are connected to a scan control circuit, a signal processing circuit, or another external circuit (not shown) at the edges of the glass substrate
104
via a TCP (Tape Carrier Package).
Now, operating principles of the two-dimensional image sensor will be explained. The photoconductive film
102
internally produces electric charge (electron-hole) when the photoconductive film
102
is irradiated with X-rays while voltage is being applied to the common electrode
103
and the storage capacitance electrode
114
. The produced electric charge moves toward either the positive or negative electrode depending on the polarity of the applied voltage and stored in the charge storage capacitance
106
.
The electric charge stored in the charge storage capacitance
106
is dischargeable through the source electrode
112
when the TFT
105
conducts in response to an input signal to the gate electrode
108
.
The pixels, since being arranged in an X-Y matrix form as described above, is capable of producing two-dimensional information of the image, by sequentially feeding the gate electrodes
108
with a signal and detecting the electric charge discharged through the source electrodes
112
for each source line.
Incidentally, the efficiency of photoelectric conversion by the photoconductive film
102
varies depending on the material composing the photoconductive film
102
. Typically, to achieve a desirable or even better efficiency, the photoconductive film
102
needs to be thick. For example, an a-Se photoconductive film
102
is deposited at a thickness of about 0.5 mm to 1.5 mm. In this case, the applied voltage should be as high as a few kilovolts.
Under these circumstances, if X-rays are shone excessively or the TFT
105
is turned off for an extended period of time, as electric charge builds up in the detecting of the image, a high voltage, which is almost equal to the applied voltage at maximum, is applied across the pixel electrode
107
. Therefore, the applied voltage of a few kilovolts may place a voltage load as high as a few kilovolts at maximum to the pixel electrodes
107
.
When this voltage load becomes greater than the tolerable voltage of the TFT
105
or the charge storage capacitance
106
, the TFT
105
or the charge storage capacitance
106
are destructed due to insulation breakdown, seriously affecting the operation of the image sensor.
Related to this problem, some measures have been devised to prevent the destruction of the TFT
105
and the charge storage capacitance
106
. Specifically, Japanese Laid-Open Patent Application No. 6-342098/1994 and other documents disclose protector capacitance; Japanese Laid-Open Patent Application No. 10-170658/1998 (Tokukaihei 10-170658; published on Jun. 26, 1998) and other documents disclose discharge of excessively stored electric charge by a protection circuit separately provided in the pixel; Brad Polischuk, et al.,
Direct Conversion Detector for Digital Mammography,
SPIE Physics of Medical Imaging, 1999, Vol. 3659, pages 417-425 (published on May 1999) and other documents disclose discharge of excessively stored electric charge by means of voltage breakdown properties of the TFT element; and PCT WO96/34416 (published on Oct. 31, 1996) and other documents disclose discharge of excessively stored electric charge by means of a double gate structure TFT.
These measures, except those involvin
Izumi Yoshihiro
Teranuma Osamu
Nixon & Vanderhye PC
Porta David
Sharp Kabushiki Kaisha
Sung Christine
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