Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-12-29
2002-07-23
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C438S096000
Reexamination Certificate
active
06423973
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1999-67854, filed on Dec. 31, 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 X-ray image sensors. More particularly, it relates to X-ray image sensors having a TFT (Thin Film Transistor) array, and to a method for fabricating the same.
2. Discussion of the Related Art
X-ray detection has been widely used for medical diagnosis. X-ray detection typically uses an X-ray film to produce a photograph. Therefore, some predetermined developing and printing procedures are required to produce the photograph.
However, digital X-ray image sensors that employ TFTs (Thin Film Transistors) have been developed. Such X-ray image sensors have the advantage that a real time diagnosis can be obtained.
FIG. 1
is a schematic, cross-sectional view illustrating the structure and operation of an X-ray image sensing device
100
. Included are 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
.
The photoconductive film
2
produces electron-hole pairs
6
in proportion to the strength of external signals (such as incident electromagnetic waves or magnetic waves). That is, the photoconductive film
2
acts as a converter that converts external signals, particularly X-rays, into electric signals. Either the electrons or the holes are then gathered by the pixel electrode
12
as electric charges. The pixel electrode
12
is located beneath the photoconductive film
2
. Which electric charges that is gathered depends on the voltage (Ev) polarity that is applied to the conductive electrode
24
by the high voltage D.C. power supply
26
. The gathered electric charges accumulate in the storage capacitor
10
, which is formed in connection with a grounding line. Charges in the storage capacitor
10
are then selectively transferred through the TFT
3
, which is controlled externally, to an external image display device that forms an X-ray image.
In such an X-ray image sensing device, to detect and convert weak X-ray signals into electric charges it is beneficial to decrease the trap state density (for the electric charge) in the photoconductive film
2
, and to decrease charge flow in non-vertical directions. Decreasing non-vertical charge flow is usually accomplished by applying a relatively high voltage between the conductive electrode
24
and the pixel electrode
12
.
Electric charges in the photoconductive film
2
are trapped and gathered not only on the pixel electrode
12
, but also over the channel region of the TFT
3
. Even during the OFF state, the electric charges trapped and gathered on the pixel electrode
12
and on the channel region of the TFT
3
induce a potential difference between the TFT
3
and the pixel electrode. This has a similar effect as the TFT
3
being in the ON state. This adversely affects the switching of the TFT
3
and increases the OFF state leakage current. Such can result in an undesired image.
FIG. 2
is a plan view illustrating a pixel of the X-ray image sensor panel. Shown are the TFT
3
, a storage capacitor “S” and gate and data lines
30
and
40
.
The gate line
30
is arranged in one direction and the data line
40
is arranged perpendicular to the gate line
30
. The TFT
3
is formed near the crossing of the gate and data lines
30
and
40
. The TFT
3
includes a gate electrode
32
, which is formed by an elongation of the gate line
30
, and a source electrode
42
, which is formed by an elongation of a data line
40
. The TFT
3
also includes a drain electrode
44
that is spaced apart from the source electrode
42
.
A ground line
52
is parallel to the data line
40
and perpendicular to the gate line
30
. The ground line
52
crosses the storage capacitor area and acts as a common electrode that is shared by adjacent pixels. A ground line contact hole
54
is formed over the ground line
52
such that a capacitor electrode
46
contacts the ground line
52
through the ground line contact hole
54
. Two or more two ground line contact holes can be formed over the ground line
52
.
The storage capacitor “S”, which stores the electric charges, is comprised of the capacitor electrode
46
, a pixel electrode
56
, and a dielectric layer (not shown) that is interposed between the capacitor electrode
46
and the pixel electrode
56
. The pixel electrode
56
extends over the TFT
3
and acts as the other capacitor electrode. In order to couple the electrons (which come from the TFT “3”) with the holes (which are stored in the storage capacitor “S”), the pixel electrode
56
is electrically connected to the drain electrode
44
via a drain contact hole
50
and via an auxiliary drain electrode
48
.
A gate pad
34
is formed at one end of the gate line
30
, and a data pad
41
is formed at one end of the data line
40
. The data pad
41
includes a data pad connector
45
that contacts the data line
40
through the first data pad contact hole
43
. Thus, the data line
40
is electrically connected to the data pad
41
.
The principle and the function of the X-ray image sensing device will now be explained.
The holes (electric charges) generated in a photoconductive film (not shown) are accumulated on the pixel electrode
56
and stored in the storage capacitor “S” with the capacitor electrode
46
.
The holes in the storage capacitor “S” are transferred to the source electrode
42
through the drain electrode
44
when the TFT
3
is turned ON. The holes arrive at an external image display device that forms an X-ray image. At this time, the ground line
52
removes the residual charges (holes) that are not transferred to the external image display device, i.e., that remain in the storage capacitor “S”. Of course, the foregoing discussion of holes is to be taken in an engineering context as holes are not physical currents.
FIGS. 3A
to
3
E are cross-sectional views, taken along line III—III of
FIG. 2
, that illustrate manufacturing processes of an X-ray image sensor panel.
Referring to
FIG. 3A
, a gate electrode
32
, a data pad
41
and a data pad connector
45
are formed on a substrate
1
by depositing and patterning a low resistant metallic material such as Aluminum (Al) or Al-alloy (for example, AlNd) using a first mask. The substrate
1
is made of a glass substrate, which is mainly used when processing is performed at a low temperature, or of a quartz glass, which has a high melting temperature and is more suitable for high temperature processing.
FIG. 3B
illustrates a manufacturing step of forming a first insulation layer
60
and semiconductor layers
65
and
63
. The first insulation layer
60
is formed at a thickness of about 4000 Å by depositing an inorganic insulation material such as Silicon Nitride (SiN
x
) or Silicon Oxide (SiO
x
). Silicon Nitride (SiN
x
) is beneficially used in a related art.
After that, the semiconductor layers are formed by depositing a pure amorphous silicon
62
and a doped amorphous silicon
64
in sequence. The CVD (Chemical Vapor Deposition) or the Ion Injection Method are beneficially used to form the doped amorphous silicon layer
64
. The CVD method is employed in a related art.
The semiconductor layer
65
and the island-shaped semiconductor layer
63
are formed by patterning the pure amorphous silicon and the doped amorphous silicon using a second mask. The island-shaped semiconductor layer
63
acts as an auxiliary electrode of a ground line that will be formed later.
Referring to
FIG. 3C
, a first data pad contact hole
43
is formed over the data pad connector
45
by patterning the first insulation layer
60
using a third mask. Then, a data line
40
, a source electrode
42
, a drain electrode
44
and a ground line
52
are formed by depositing and patterning a second metal, such as Chrome (Ch) or a Cr
Choo Kyo-Seop
Park June-Ho
Hannaher Constantine
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
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