Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-03-24
2002-10-08
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370090, C250S367000, C250S363020, C250S370110, C250S385100
Reexamination Certificate
active
06462344
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
This application claims the benefit of Korean Patent Application No. 1999-10397, filed on Mar. 25, 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 detector, and more particularly to an X-ray image detector fabricated utilizing a Thin Film Transistor (TFT) array process and a method for fabricating the same.
2. Description of Related Art
The widely used X-ray detection method for medical diagnosis is such that an X-ray detecting film is used to produce a photograph and some predetermined printing procedure is required to obtain the result.
However, digital X-ray detectors employing TFTs have been developed recently due to the development of semiconductor technology. This X-ray image detector has an advantage that a real time result for diagnosis can be obtained immediately after photographing because it uses a TFT as a switch.
The above-described X-ray image detector and a TFT-LCD have many characteristics in common in Table 1.
TABLE 1
X-ray image detector
TFT-LCD
USE
X-ray detection
Display of images
PANEL COMPOSITION
TFT panel +
TFT + liquid
photoconductive
crystal +
film +
color fllter
Upper electrode
USE OF TFT
Switch for
Switch for applying
releasing signals
signals
DATA LINE
X-ray signal detection
Application of
Line
signal
voltages to pixels
CAPACITY OF
More than 2.1 pF
0.2 to 0.3 pF
STORAGE CAPACITOR
MATERIAL OF
ITO/ITO
A metal/ITO
STORAGE ELECTRODE
(indium tin oxide)
OPENING RATIO
85%
60 to 70%
As shown in Table 1, the opening ratio of an X-ray image detector is larger than that of a TFT-LCD by 15 to 25%. This is because the metal line of a TFT-LCD is covered with black matrix to prevent the deterioration of picture quality. However, in an X-ray image detector, as the total surface of the ITO pixel electrode corresponds to the opening part, its opening ratio is superior to that of a TFT-LCD.
FIG. 1
is a schematic cross-sectional view illustrating the structure and operation of X-ray image detector
100
which comprises lower substrate
1
, thin film transistor
3
, storage capacitor
10
, pixel electrode
12
, photoconductive film
2
, protection film
20
, conductive electrode
24
and high voltage D.C. (direct current) power supply
26
.
Photoconductive film
2
produces internal electric signals, i.e. pairs of electron (e) and holes (h), in proportion to the strength of external signals such as incident electromagnetic waves or magnetic waves. The photoconductive film
2
serves as a conversion to detect external signals, particularly X-rays, and convert them to electric signals. Electron-hole pairs (
6
) are gathered in the form of electric charges at pixel electrode
12
located beneath the photoconductive film
2
by a voltage (E
v
) applied to conductive electrode
24
by the high voltage D.C. power supply
26
, and then is stored in storage capacitor
10
formed in connection with a common electrode grounded externally. Charges stored in the storage capacitor
10
are transferred by TFT
3
, controlled externally, to an external image display device and forms X-ray images.
In an X-ray image detector, to detect and convert even a weak X-ray signal into electric charges, it is required to decrease the number of electric charges trapped in the photoconductive film
2
, and to decrease the current in non-vertical directions by means such as applying a high voltage (more than 10V/&mgr;m) in the vertical directions between conductive electrode
24
and pixel electrode
12
.
Electric charges in the photoconductive film
2
produced by X-ray energy are trapped and gathered on a protection film (not illustrated),which protects the channel part of the TFT
3
, as well as on the pixel electrode. This electric charges trapped and gathered induce electric charge into the channel region in the upper part of TFT
3
, producing a high leakage current even when TFT
3
is in an “off” state, thus inhibiting switching operation of TFT
3
.
Moreover, electric signals stored in the storage capacitor
10
are discharged externally due to the high leakage current in the “off” state, as a result of which, the desired image can not be obtained.
FIG. 2
is a cross-sectional view schematically illustrating a conventional X-ray image detector. U.S. Pat. No. 5,498,880 discloses one example of this kind of structure wherein pixel electrode
12
is extended to cover the upper part of TFT
3
, (so called “mushroom structure”) to prevent the trapping of electric charges on the upper part of TFT
3
, induced from the electric charges produced in photoconductive film
2
by X-ray.
The manufacture of the conventional X-ray image detector 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 the 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 storage electrode
40
. ITO (indium tin oxide) is most commonly used as the transparent conductive material.
After forming the first storage electrode
40
, second insulation film
34
b
is formed on the first insulation film
34
a
and first storage electrode
40
. At a predetermined position of the second insulation film
34
b
on the first storage 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
, drain electrode
32
and ground line
42
. The source/drain metal is usually aluminum which has a low resistance and good deposition properties. Protection layer
46
is formed after the formation of
33
,
32
and
42
, in order to protect TFT
3
.
In the protection layer
46
contact holes are formed on the source electrode
33
for contact with a second storage electrode which will be formed later. Then, the protection layer
46
formed in the upper part of the first storage electrode
40
is etched out except on ground lines
42
for increasing the capacity of the storage capacitors. In the described structure, the practical charging capacity corresponds to the portion labeled C
st
in FIG.
2
. Now, ITO is deposited and patterned to form a second storage electrode
12
which serves as a pixel electrode, and a photoconductive film
2
is formed by deposition on the overall substrate
1
. The later procedures are abbreviated here.
In an X-ray image detector adopting the so-called “mushroom structure” as described above, as electric charges produced by X-ray energy gather on the pixel electrode
12
of the storage capacitor, the electric potential of the storage capacitor increases. The increased potential causes an increased capacity of a parasitic capacitor which is formed between the “mushroom” associated with the pixel electrode and TFT
3
.
The capacity of a parasitic capacitor is inversely related to the thickness of the protection layer
46
to protect the channel part in the upper part of TFT
3
such that the capacitance increases as the thickness of the protection layer
46
is reduced, 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 a parasitic capacitor of TFT
3
may be decreased if the thickness of an organic insulation film used as dielectric material of storage capacitor is increased to more than 1.5 &mgr;m, the rear surface of TFT
3
under an acrylic protection film of the conventional X-ray image detector can have about 1 &mgr;m thickness, which allows increase of leakage current in an “off” state.
Furthermore, second insulation film
34
b
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 detector
Joo Insu
Jung Yuho
Hannaher Constantine
Israel Andrew
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
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