Storage capacitor structure for LCD and OELD panels

Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Recrystallized semiconductor material

Reexamination Certificate

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C257S072000, C257S359000, C257S070000, C257S052000, C257S059000

Reexamination Certificate

active

06833561

ABSTRACT:

PRIORITY CLAIM
The present application claims priority from Korean patent application no. 2001-68219, filed Nov. 2, 2001, which is incorporated by reference.
1. Technical Field
The present invention relates to a structure and a fabrication method of a storage capacitor used in pixel region of a liquid crystal display (LCD) panel or an organic electronic luminescent display (OELD) panel. More particularly, the invention relates to a technique of simultaneously forming a crystalline silicon pixel transistor and a storage capacitor using metal induced lateral crystallization (MILC) in pixel region of a thin film transistor (TFT) panel for an LCD or an OELD.
2. Background of the Invention
FIG. 1
schematically illustrates a TFT panel
10
for an LCD including a pixel region
11
and a driving circuit region
12
located in the periphery of the pixel region. When the amorphous silicon layer of a TFT formed on a substrate is crystallized by MILC, the electron mobility of the silicon layer is significantly increased. Using the crystallized silicon layer, a plurality of pixel arrays including a pixel transistor and a storage capacitor may be formed in the pixel region
11
. At the same time, driving circuit elements may be formed in the driving circuit region
12
. For polycrystalline silicon TFT LCD, hybrid driving methods are widely used. In a hybrid type LCD panel, certain analog circuits such as OP amplifier and DA converter, which are difficult to fabricate with polycrystalline silicon, are provide by using separate integrated circuits, and switching elements such as multiplexer are directly formed on the substrate.
FIG. 2
is an equivalent circuit diagram of a pixel unit formed in the pixel region of the LCD TFT panel
10
illustrated in FIG.
1
. Each unit pixel includes a data bus line (Vd); a gate bus line (Vg); a pixel TFT comprising a gate connected to the gate bus line, a source and a drain respectively connected to the data bus line and a pixel electrode; a storage capacitor (Cst)
22
for maintaining the state of the signal applied to a pixel TFT
22
; and a liquid crystal (C
LC
) connected in parallel with the storage capacitor. The storage capacitor and the liquid crystal are connected to a common electrode (V
COM
)
24
. When a unit pixel is selected by the gate bus signal and voltage is applied by the data bus signal, a storage capacitor
22
connected to a drain of a pixel transistor
21
stores electric charge and maintains the voltage applied to the liquid crystal until next signal is applied. Without a storage capacitor, a driving voltage applied by a pixel transistor may not be maintained until the next signal period. Then, continuous display may not be performed.
OELD panel has a condenser structure including a surface glass comprising a transparent glass and transparent electrode; a metal electrode used as a cathode; and an organic luminescent layer interposed between the transparent electrode and the metal electrode. When voltage is applied between the electrodes, the organic luminescent layer emits light through the surface glass. LCD panels including TFT LCD panel have several limitations such as low response speed, narrow vision angle and high power consumption due to a backlight unit. Because OELD panel is a self-light-emitting device, it has advantages of high response speed, high luminescence, low profile structure and low power consumption.
FIG. 3
is a schematic diagram of an OELD TFT panel
30
including pixel region
31
and a driving circuit region
32
formed in the periphery of the pixel region. When the silicon active layer of the TFT is crystallized by MILC, the electron mobility of the active layer is increased. Thus, using MILC crystallization technique, driving circuit elements of high operation speed can be simultaneously formed in the driving circuit region when forming a plurality of pixel arrays including addressing transistor, storage capacitor and pixel driving transistor in the pixel region
31
. As the case of LCD panel, hybrid driving methods are frequently used for driving OELD TFT panels.
FIG. 4A
is an equivalent circuit diagram of a unit pixel formed in the pixel region of a voltage-driven type OELD TFT panel
30
. Each unit pixel includes a data bus line (Vd); a gate bus line (Vg); and an addressing (switching) TFT comprising a gate connected to the gate bus line and a source and a drain connected to the data bus line. The drain of the addressing TFT
41
is connected to the gate of a pixel driving TFT
43
for receiving a reference voltage (Vdd) and providing a driving voltage (Vc) to organic luminescent material layer. A storage capacitor
42
for maintaining the signal applied to the gate of the pixel driving TFT is also connected in parallel with the pixel driving TFT
43
. Since TFT LCD is not a self-light-emitting type device, only one pixel TFT is used in a unit pixel to provide a voltage to a pixel electrode. However, in OELD, data signal may not provide a voltage required to induce the light emission of the organic material. Thus, it has to use a separate pixel driving TFT
43
receiving the output signal of the addressing TFT
41
as a gate signal.
FIG. 4B
illustrates an example of the equivalent circuit diagram of a unit pixel in the pixel region of a current-driven type OELD TFT panel
30
. A unit pixel of a current-driven type OELD TFT panel includes two addressing TFT's
44
,
45
, two pixel driving TFT's
47
,
48
and one storage capacitor
46
. A first addressing TFT
44
is turned on by a signal of a first gate bus line (Vg
1
) to receive a signal of the data bus line (Vd). A second addressing TFT
45
is turned on by a signal of a first gate bus line (Vg
2
) and provides the output of the first addressing TFT
44
to the gates of a pair of pixel driving TFT's
47
,
48
and to the storage capacitor
46
. When electric charge is accumulated in the storage capacitor
46
after the first addressing TFT
44
and the second addressing TFT
45
are turned on, the voltage created in the storage capacitor is applied to the gates of the first and the second pixel driving TFT's
47
,
48
to turn on the pixel driving TFT's. The voltage applied by the storage capacitor is maintained even when the second addressing TFT is turned off. Thus the turn on state of the pixel driving TFT's
47
,
48
is maintained until the next signal period and they continue to provide the driving current to the unit pixel.
As can be seen from
FIGS. 2
,
4
A and
4
B, a storage capacitor for LCD TFT panel or OELD TFT panel is connected to an LCD pixel TFT or to the drain of an OELD addressing TFT (a second addressing TFT for an current-driven type).
FIGS. 5A and 5B
are a plan view and a sectional view of an LCD pixel TFT or an OELD addressing TFT including a polycrystalline silicon active layer crystallized by MILC, which is connected to a storage capacitor simultaneously formed with the TFT.
FIGS. 5A and 5B
show a thin film transistor in the left-hand side and a capacitor structure in the right-hand side. The TFT is used as a pixel TFT in an LCD panel and as an addressing TFT in a unit pixel of an OELD panel. The TFT may be used as a second addressing TFT
45
in current-driven type OELD panel as shown in FIG.
4
B. In
FIGS. 5A and 5B
, a drain of the TFT is directly connected to a silicon layer of the capacitor. In actual pixel layout of LCD or OELD, however, they may not be physically connected with each other. Instead, they may be electrically connected with each other by wire.
On the transparent substrate
51
, a buffer layer
52
for preventing diffusion of impurities from the substrate
51
is formed. On the buffer layer, an amorphous silicon layer
53
is patterned and a gate insulating layer
54
and a capacitor dielectric layer
55
are formed on the patterned silicon layer. Thereafter, a gate electrode layer
56
and a capacitor electrode
57
are formed on the gate insulating layer and the dielectric layer, respectively. As such, a TFT structure including an amorphous silicon layer
53
, gate

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