X-ray detector and method of fabricating therefore

Details X-ray detector and method of fabricating therefore X-ray detector and method of fabricating therefore

2002-03-12

2004-05-18

Gagliardi, Albert (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S370080, C257S428000

Reexamination Certificate

active

06737653

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to X-ray detectors. More particularly, it relates to Thin Film Transistor (TFT) array substrates for use in X-ray detectors.
2. Description of Related Art
A widely used method of medical diagnosis is the X-ray film. As such films produce photographic images, time consuming film-processing procedures are required to obtain the results. However, digital X-ray sensing devices (referred to hereinafter as X-ray detectors) employing thin film transistors have been developed. Such X-ray sensing devices have the advantage of providing real time diagnosis.
FIG. 1
is a schematic cross-sectional view illustrating the structure and operation of an X-ray detector
100
according to a conventional art. 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). That is, the photoconductive film
2
acts as a converter that converts external signals, particularly X-rays, into electric signals. When an external voltage Ev is applied across a conductive electrode
24
, that voltage causes the electron-hole pairs
6
in the photoconductive film
2
to separate such that X-ray induced electrical charges accumulate on the pixel electrode
12
. Thus, either the electrons or the holes are then gathered by the pixel electrode
12
as electric charges.
As shown in
FIG. 1
, the pixel electrode
12
is located beneath the photoconductive film
2
, and the electric charges that are gathered depend 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 are accumulated 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 a thin film transistor (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. Further for sensing the weak X-ray signals, it is also essential to decrease leakage current when the TFT
3
is turned off.
FIG. 2
is a plan view illustrating one pixel of an array substrate for an X-ray detector according to the conventional art. A gate line
30
is arranged in a transverse direction and a data line
40
is arranged in a longitudinal direction. A thin film transistor (TFT)
3
acting as a switching element is formed near each crossing of the gate and data lines
30
and
40
. A storage capacitor
10
, which is arranged in a pixel region defined by a pair of gate line
30
and data line
40
, includes a capacitor electrode
46
, a pixel electrode
56
and a dielectric layer. The capacitor electrode
46
acts as not only a first electrode of the storage capacitor
10
but also a common electrode by way of being connected to its neighboring capacitor electrode. The pixel electrode
56
corresponds to the capacitor electrode
46
to act as a second electrode of the storage capacitor
10
. Although not shown in
FIG. 2
, a dielectric layer is interposed between the capacitor electrode
46
and the pixel electrode
56
. The pixel electrode
56
gathers the electric charges generated in the photoconductive film in order to keep the electric charges in the storage capacitor
10
. Furthermore, the pixel electrode
56
is electrically connected to a drain electrode
44
of the TFT
3
via a drain contact hole
50
for transmitting the electric charges to the data line
40
through the TFT
3
.
The operation of the X-ray detector described above is as follows. The electronic charges generated in the photoconductive film are gathered in the pixel electrode
56
and stored in the storage capacitor
10
having the capacitor electrode
46
. The stored electronic charges are then moved to a source electrode
42
through the pixel and drain electrodes
56
and
44
by the operation of the TFT
3
. Thereafter, the electronic charges move through the data line
40
and finally display the images in the external image display device.
The fabrication steps of the array substrate illustrated in
FIG. 2
will be explained with reference to
FIGS. 3A
to
3
G, which are cross-sectional views taken along line III—III of FIG.
2
.
Referring to
FIG. 3A
, a first metal layer is formed on a substrate
1
by depositing a metallic material such as Aluminum (Al) or Al-alloy (e.g., AlNd). A gate line (see reference element
30
of
FIG. 2
) and a gate electrode
32
that extends from the gate line are then formed by patterning the first metal layer. As a material for the substrate
1
, either a quartz having a high melting point or a glass having a relatively low melting point can be used. Since the glass is cheap and has a low melting point rather than the quartz, the glass is more adequate for the substrate that is used in under the low temperature process.
In
FIG. 3B
, a first insulation layer
60
is deposited to a thickness of 4000 angstroms (Å) over the substrate
1
and over the first patterned metal layer. The first insulation layer
60
can be comprised of an inorganic substance, such as Silicon Nitride (SiN
X
) or Silicon Oxide (SiO
X
). A pure amorphous silicon (a-Si:H) layer and a doped amorphous silicon (n
+
a-Si:H) layer are sequentially formed on the first insulation layer
60
. Those silicon layers are then patterned to form an active layer
62
and an ohmic contact layer
64
. CVD (Chemical Vapor Deposition) or the Ion Injection Method can beneficially be used to form the doped amorphous silicon layer.
FIG. 3C
shows a step of forming a source electrode
42
, a drain electrode
44
, and a capacitor electrode
46
. First, a second conductive metal layer is deposited on the first insulation layer
60
to cover the active layer
62
and the ohmic contact layer
64
. The second conductive metal layer is then patterned to simultaneously form the source electrode
42
, which extends from the data line
40
over the gate electrode
32
; the drain electrode
44
, which is spaced apart from the source electrode
42
and over the gate electrode
32
; and the capacitor electrode
46
, which is the first electrode of the storage capacitor
10
(see FIG.
2
). Thereafter, a portion of the ohmic contact layer
64
on the active layer
62
is then etched to form a channel region using the source and drain electrodes
42
and
44
as masks. Thus, the TFT
3
(see
FIG. 2
) is complete.
Next in
FIG. 3D
, a planarizing protection layer
66
that acts as a dielectric layer in the storage capacitor is formed over the TFT and on the capacitor electrode
46
. The planarizing protection layer
66
is then patterned to form a drain contact hole
50
to expose a portion of the drain electrode
44
. The planarizing protection layer
66
is made of an organic material, such as benzocyclobutene (BCB) or acryl-based resin, thereby planarizing the surface of the substrate
1
having the TFT and capacitor electrode
66
.
Referring now to
FIG. 3E
, a pixel electrode
56
, which connects to the drain electrode
44
via the drain contact hole
50
, is formed by depositing and patterning a transparent conductive material such as ITO (indium-tin-oxide) or IZO (indium-zinc-oxide).
Now referring to
FIG. 3F
, a photoconductive film
2
and a protection layer
20
are sequentially formed on the pixel electrode
56
. As described hereinbefore, the photoconductive film
2
converts the external signals, particularly X-rays, into the electrical signals. The photoconductive film
2
is benef

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