Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-03-28
2002-06-18
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
Reexamination Certificate
active
06407393
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
This application claims the benefit of Korean Patent Application No. 1999-11516, filed on Apr. 1, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an X-ray image sensor, and more particularly to an X-ray image sensor fabricated utilizing a TFT (Thin Film Transistor) array process and a method for fabricating the same.
(2) Description of Related Art
An X-ray detection method that has been widely used for medical diagnosis is such that an X-ray detecting film is used to produce a photograph and some predetermined printing procedures are required to obtain the result.
However, digital X-ray detectors employing TFT (Thin Film Transistor) have been developed recently due to the development of semiconductor technology. This X-ray image sensor has an advantage that a real time diagnosis can be obtained immediately after photographing because it uses a TFT as switching element.
FIG. 1
is a schematic cross-sectional view illustrating the structure and operation of an X-ray image sensing device
100
which comprises a lower substrate
1
, a thin film transistor
3
, a storage capacitor
10
, a pixel electrode
12
, a photoconductive film
2
, a protection film
20
, a conductive electrode
24
and a high voltage D.C. (direct current) power supply
26
.
Photoconductive film
2
produces internal electric signals, i.e. electron-hole pairs in proportion to the strength of external signals such as incident electromagnetic waves or magnetic waves. That is, the photoconductive film
2
serves as a converter to detect external signals, particularly X-rays and convert them into electric signals. Either electrons or holes are gathered at pixel electrode
12
located beneath the photoconductive film
2
depending on a voltage (Ev) applied to the conductive electrode
24
by the high voltage D.C. power supply
26
, and then are stored in storage capacitor
10
formed in connection with a ground line grounded externally. Charges stored in the storage capacitor
10
are transferred to the TFT
3
, controlled externally, to an external image display device and form X-ray images.
In an X-ray image sensing device, to detect and convert even the weakest X-ray into electric charges, it is required to decrease the trap state density for the electric charge in the photoconductive film
2
and to decrease the amount of current in the non-vertical direction by applying a relatively high voltage (more than 10V/&mgr;m) in the vertical direction between the conductive electrode
24
and pixel electrode
12
.
Electric charges in the photoconductive film
2
produced by X-ray energy are trapped and gathered on the pixel electrode
12
. Even during the OFF state of the TFT
3
, electric charges trapped and gathered on the pixel electrode
12
, particularly over the channel region of the TFT
3
, induce a potential difference between the TFT
3
and the pixel electrode, which causes the same effect in the TFT
3
as ON state. That is, the pixel electrode
12
functions as a gate of the TFT
3
, thus, adversely affecting the switching operation of the TFT
3
and increasing leakage current through the TFT
3
even when in an OFF state. This results in an undesired image.
FIG. 2
is a cross-sectional view schematically illustrating a conventional X-ray image sensor. U.S. Pat. No. 5,498,880 discloses one kind of structure wherein the pixel electrode
12
extends to cover the upper part of TFT
3
(a so called “mushroom structure”) to prevent the trapping of electric charges in the upper part of TFT
3
induced from the electric charges produced in photoconductive film
2
by X-ray energy.
The manufacture of the conventional X-ray image sensor will be described hereinafter referring to FIG.
2
.
First, substrate
1
is deposited with a metal and patterned to form a gate electrode
31
. Then, SiNx is deposited thereon in a thickness of about 100 nm to form a first insulation film
34
a
. After the formation of film
34
a
, a transparent conductive material is deposited and patterned to form a first capacitor electrode
40
. ITO (indium tin oxide) is most commonly used as the transparent conductive material.
After forming the first capacitor electrode
40
, a second insulation film
34
b
is formed on the first insulation film
34
a
while covering the first capacitor electrode
40
. At a predetermined position of the second insulation film
34
b
on the first capacitor electrode
40
, a contact hole
41
is formed for contact with a ground line
42
that will be formed later. Thereafter, a source/drain metal material is deposited and patterned to form a source electrode
33
, a drain electrode
32
and the ground line
42
. The source/drain metal is usually aluminum that has a low resistance and good deposition properties. Protection film
46
is formed after the formation of
33
,
32
and
42
, in order to protect TFT
3
from external impact or humidity.
In the protection layer
46
on the source electrode
33
, contact holes are formed for contact with the pixel electrode that will be formed later. Then, the protection layer
46
formed in the upper part of the first capacitor electrode
40
, except on ground line
42
, is etched out in order to decrease the thickness of the dielectric layer and increase the capacity of the storage capacitors. Then, ITO is deposited and patterned to form a pixel electrode
12
which serves as second capacitor electrode, and a photoconductive film
2
is formed by deposition over the whole substrate
1
. The later procedures are abbreviated here.
In an X-ray image sensor adopting the so-called “mushroom structure” as described above, electric charges produced by X-ray energy gather on the pixel electrodes and there is formed a parasitic capacitor between pixel electrode and TFT.
The capacity of a parasitic capacitor has an inverse relationship with respect to the thickness of the protection film for protecting the channel part in the upper part of TFT such that it increases as the thickness is decreased, inducing a large amount of charges to the channel part, which increases the amount of leakage current even if TFT is in an “off” state and deteriorates its switching operation.
Though the capacity of the parasitic capacitor of the TFT may be decreased when the thickness of the protection film made of acrylic on the TFT is increased, in the above structure there is a limit to reduce the capacity of the parasitic capacitor, since the dielectric constant of acrylic is relatively high.
Furthermore, the second insulation film used as a dielectric of the storage capacitor is formed to be thin, with a thickness of about 200 nm in a conventional X-ray image sensor. Therefore the second insulation film may be etched out or overetched while etching the organic insulation film as a protection film in order to form a contact hole for contacting the pixel electrode, or the second capacitor electrode and the second insulating film to decrease the thickness of the dielectric layer. This causes an electrical short of the first and second capacitor electrodes and accordingly the number of point defects is increased, leading to low yield. The present invention has been developed as a result of the continuous research done by the inventors for solving the above-described problems.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an X-ray image sensor in which the “off” electric current is decreased by lowering the leakage current of a TFT.
Another object of the present invention is to provide an X-ray image sensor which can prevent electrical shorts between gate and source/drain electrodes and a capacitor electrode.
A further object of the invention is to provide an X-ray image sensor which can decrease processing errors which may occur in the production process.
In order to achieve the above objects, the present invention provides, in a first aspect, an X-ray image sensor which comprises: a photoelectri
Choi Jae Beom
Jeong Young Sik
Kim Chang Won
Kim Chang Yeon
Yoon Jung Kee
Hannaher Constantine
LG. Philips LCD Co. Ltd.
Long Aldridge & Norman LLP
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