Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2000-08-03
2002-04-16
Christianson, Keith (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
Reexamination Certificate
active
06373116
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to two-dimensional image detectors capable of detecting images by means of X-rays and other kind of radiation and visible, infrared, and other light.
BACKGROUND OF THE INVENTION
Conventionally known two-dimensional image detectors for detecting images of radiation, such as X-rays, or light, such as visible or infrared light, form an image by collecting and reading electric charges generated in a photosensitive semiconductor layer by means of pixels located on an active matrix substrate such as the one used in a conventional type of liquid crystal display device. For the purpose of increasing the per-pixel fill factor (aperture ratio), some conventional two-dimensional detectors have a so-called roof structure in which pixel electrodes are separated by an insulating layer from the address lines (electrode wires) and TFT elements that are provided in turn on the active matrix substrate.
More specific description of the structure can be found in Similarities between TFT Arrays for Direct-Conversion X-ray Sensors and High-Aperture AMLCDs, by W. den Boer, et al., SID 98 DIGEST, PP.371-374, 1998; and U.S. Pat. No. 5,780,871 (Date of patent: Jul. 14, 1998).
FIG. 9
is a cross-sectional view schematically showing an arrangement of a pixel in a two-dimensional image detector having a conventional roof-structured active matrix substrate.
As shown in
FIG. 9
, in the base structure of the two-dimensional image detector, there are provided a roof-structured active matrix substrate
101
, a photoconductor film
102
, and a common electrode
103
, forming sequential layers in this order.
The roof-structured active matrix substrate
101
includes a glass substrate
104
, a TFT (Thin Film Transistors)
105
, an electric charge storage capacitance (Cs)
106
, and a pixel electrode
107
. The TFT
105
acts as a switching element. The pixel electrode
107
caps an insulating layer
115
which in turn covers the TFT
105
and the electric charge storage capacitance (Cs)
106
.
The TFT
105
is constituted by a gate electrode
108
, a gate insulating film
109
, an a-Si film (i layer)
110
, a-Si film (n
+
layer)
111
, a source electrode
112
, and a drain electrode
113
. The electric charge storage capacitance (Cs)
106
is constituted by a storage capacitance electrode (Cs electrode)
114
, a gate insulating film
109
, and a drain electrode
113
that also acts as a storage capacitance electrode forming a pair with the storage capacitance electrode (Cs electrode)
114
.
The insulating layer
115
is interposed so as to electrically insulate the pixel electrode
107
from the electrode wires (the gate electrode
108
and the source electrode
112
), the TFT
105
, the electric charge storage capacitance (Cs)
106
. The pixel electrode
107
and the drain electrode
113
are electrically connected in a contact hole
116
formed through the insulating layer
115
.
The photoconductor film
102
is composed of a semiconductor material that generates electric charges (electrons, holes) when irradiated with radiation such as X-rays or light such as visible light.
Now, principles in the operation of the two-dimensional image detector will be discussed.
Electric charges (electrons, holes) are generated in the photoconductor film
102
when radiation such as X-rays or light such as visible light is projected to the photoconductor film
102
with voltage being applied across the common electrode
103
and the storage capacitance electrode (Cs electrode)
114
. The generated electric charges move toward either the positive or negative electrode according to the sense of the apply voltage and are stored in the electric charge storage capacitance (Cs)
106
. The electric charges stored in the electric charge storage capacitance (Cs)
106
are available for output through the source electrode
112
if the TFT
105
is changed to an open state by an input signal to the gate electrode
108
.
The electrode wires (gate electrode
108
and source electrode
112
), the TFT
105
, and the electric charge storage capacitance (Cs)
106
, when provided to form a X-Y matrix, are capable of scanning input signals to the gate electrodes
108
sequentially row by row and thereby obtain image information in two dimensions.
In two-dimensional image detectors including the aforementioned roof-structured active matrix substrate, an insulating layer is interposed between the pixel electrodes and the address lines (electrode wires) to provide insulation between them.
FIG. 9
shows the insulating film
115
as an example of such an insulating layer. The insulating layer may be composed of a material such as, SiO
x
, SiN
x
, Al
2
O
3
, polyimide, or an acrylic resin; however, the level of insulation these materials offer varies from material to material for the following reasons.
Resin films can be formed using a spin coating, film lamination, or other similar technique. SiO
x
and SiN
x
. films are costly, in comparison with resin films, due to the use of CVD (Chemical Vapor Deposition) in the manufacturing process. Resin films can be readily fabricated so as to make a flat surface on them using a spin coating technique, etc. By contrast, SiO
x
and SiN
x
films deposited using a CVD technique are inevitably affected by irregularities in the underlying layer, and will have a surface which is far from being flat in a satisfactory manner. In two-dimensional image detectors, irregularities in the surface of an insulating layer adversely affect the photoconductive layer deposited on the active matrix substrate, which undesirably degrades its detection performance. Thus, resins are the preferred material to form an insulating layer with a flat surface.
In two-dimensional image detectors, a parasitic capacitance appears where the address line (electrode wire) is placed on the top of on a pixel electrode, which is a major factor causing noise in signals. To reduce the signal noise, the parasitic capacitances should be reduced. A preferred insulating layer is therefore a thick one as long as other conditions allow. CVD techniques are hardly capable of forming a film thicker than 1 &mgr;m; by contrast, thick resin films are readily formed by spin coating. Further, a typical resin have a low dielectric constant, and allows reduction of the parasitic capacitances.
Further, contact holes should be provided in the insulating layer, in which the pixel electrodes are connected to the drain electrodes. The contact holes are formed using a photolithography technique. In view of these facts, the insulating film composed of an acrylic resin or other photosensitive material is convenient, because such an insulating film can be subjected to a photolithography process to form the contact holes, without applying and etching resist, and contributes to speedy processing in comparison with non-photosensitive materials.
For these reasons, the insulating layer is preferably made of an acrylic or other similar resin with a low dielectric constant and a satisfactory level of photosensitivity, and fabricated using a spin coating technique. To reduce the parasitic capacitances, the insulating layer is preferably made of a material with a low dielectric constant.
However, the following problems entail if a conventional two-dimensional image detector has a roof-structured active matrix substrate with an insulating layer made of the acrylic or other similar resin.
If the resin insulating layer included in a conventional roof-structured active matrix substrate is in direct contact with ambient air along the edges of a pixel area, degradation of the insulating layer material possibly greatly affects the reliability of the device. Specifically, we are concerned about ambient humidity among other factors, which is a likely cause to adversely affect the reliability. In general, acrylic and other similar resins are not durable in the presence of humidity; therefore, if composed of such a resin, the insulating layer peels off and inevitably degrades or otherwise changes in properties with time, starting wher
Izumi Yoshihiro
Teranuma Osamu
Christianson Keith
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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