Display panel

A display panel comprises a substrate having disposed thereon a plurality of display elements, each display element comprising a pixel electrode disposed on the substrate; a scan electrode disposed on the substrate separate from the pixel electrode; two display drive elements, one disposed over a first part of the pixel electrode and the other disposed over a first part of the scan electrode; an insulating layer extending over at least a second part of the pixel electrode and the two display drive elements and having at least three openings formed therein, two of the openings being coincident with and exposing a portion of each of the display drive elements and a third opening being disposed so as to expose a portion of the second part of the pixel electrode; a first connecting layer disposed over the insulating layer and forming an electrical connection through the windows in the insulating layer between the exposed portion of the display drive element disposed over the scan electrode and the exposed portion of the pixel electrode; and a second connecting layer disposed over the insulating layer and forming an electrical connection between the portion of the display drive element disposed over the pixel electrode exposed through the opening in the insulating layer and an exposed part of the scan electrode.

BACKGROUND OF THE INVENTION 
The present invention relates to an image display panel such as a liquid 
crystal display panel of active matrix type. More particularly, it relates 
to an image display panel which is made up of a substrate having an array 
of display elements thereon; each display element comprising a pixel 
electrode to display picture elements and a scan electrode to apply a 
display voltage, and display drive elements connected to the pixel 
electrode and scan electrode. 
Recently display panels, especially liquid crystal display panels, have 
come into practical use for the display of television images, although the 
size is still somewhat limited. For the display panel to display images as 
large as those of conventional CRTs, it must have a larger area and higher 
display density than before. A display panel of sufficiently large area 
and high display density requires several hundred pixels in each of the 
horizontal and vertical directions, which is equivalent to tens of 
thousands of pixels on the entire surface of the panel. If these pixels 
are to be driven by their corresponding external circuits, the drive 
circuits would be very complex and expensive. This disadvantage is 
eliminated in the case of an active matrix system. According to this 
system, each pixel is provided with a simple display drive element which 
has been previously built into the substrate of the display panel. The 
display drive element built into the substrate needs to be of thinfilm 
type. The known display drive elements include metalinsulator-metal (MIM) 
elements, thin-film transistors (TFT), and a pair of diodes connected in 
parallel with opposite polarities. The display drive element based on 
diode pairs made of amorphous silicon thin film is considered to be 
promising on account of its low cost and uniform switching 
characteristics. 
No matter what type of display drive element might be used, the decided 
disadvantage of the display panels of large area and high display density 
is that their yields are low. A display panel has tens of thousands of 
pixels on a substrate, as mentioned above. This means that a display panel 
needs hundreds of thousands of display drive elements in total because 
each display drive element of diode pair type needs two diodes, and a 
color display panel needs three times as many display drive elements as a 
monochrome display panel. It is very difficult to produce all of these 
elements completely free of defects. Although the defects which are liable 
to occur in the display drive elements differ from one type to another, 
defects occur less frequently than expected in the amorphous silicon layer 
in the case where the display drive elements are amorphous silicon diode 
pairs. Rather, defects occur frequently in the connections of pixel 
electrodes and scan electrodes. These defects lead to the short-circuit of 
display drive elements and, more frequently, the disconnection of such 
elements. FIG. 3 illustrates how defects occur in the conventional display 
panel that employs diode pairs as the display drive elements. 
FIG. 3(a) is an enlarged plan view showing a part of a display in which a 
diode pair 31, 32 for one pixel is provided. FIGS. 3(b) and 3(c) are 
sectional views taken in the direction of arrows B--B and C--C 
respectively, in FIG. 3(a). FIG. 3(a) shows pixel electrode 10 
corresponding to diode 31 and 32, and an adjacent pixel electrode 11, as 
well as a scan electrode 20 to apply a display voltage to a plurality of 
pixel electrodes arranged in the vertical direction in the figure. The 
pixel electrodes and the scan electrode 20 are formed on the entire 
surface of a colorless transparent glass substrate 1 from a transparent 
conductive metal oxide layer such as indium-tin oxide (ITO) having a 
thickness of between hundreds and thousands of angstroms by electron-beam 
evaporation or sputtering. On the metal oxide layer are formed diodes 31, 
32 in the following manner. A light-shielding layer 30a, which is a 
500-2000 .ANG. thick Cr layer is formed by sputtering. On the 
light-shielding layer 30a is grown an amorphous silicon layer 30 b of pin 
structure to a thickness of 0.5 to 1 .mu.m by plasma CVD method. On the 
amorphous silicon layer 30b is formed a Cr light-shielding layer 30c. The 
thus formed amorphous silicon layer of triple layer structure containing 
light-shielding layer undergoes reactive ion etching by photolithography 
method except those parts which become diodes 31, 32. Those parts which 
become diodes 31,32 remain unetched as shown in FIG. 3(a). The previously 
deposited ITO film undergoes chemical etching by photolithography to form 
the pixel electrode 10 and scan electrode 20 of desired pattern. 
Then a 500-2000.ANG. thick insulation layer 61 of silicon nitride as a 
protective film is formed by CVD method on the entire surface. This layer 
undergoes etching by photolithography to form a pattern which covers the 
two diodes 31, 32 as shown in FIG. 3(a). At the same time, a window 61a is 
formed in the insulation layer on the top of each diode. This patterning 
is accomplished by gas etching. The light-shielding layer 30c under the 
window 61a prevents the amorphous silicon layer 30b from being etched by a 
reactive gas. The diodes 31, 32 are connected by an aluminum layer 
according to the usual practice. An aluminum layer having a thickness of 
thousands of angstroms is formed by sputtering such that it comes into 
conductive contact with the light-shielding layer 30c as the upper 
electrode layer under the window 61a. The aluminum layer is etched by 
photolithography to form the connection layers 41, 42 having a pattern as 
shown in FIG. 3(a). The connection layer 41 connects the top electrode of 
the diode 31 formed on the drive electrode 20 to the pixel electrode 10, 
and the connection layer 42 connects the top electrode of the diode 32 
formed on the pixel electrode 10 to the scan electrode 20. Thus the 
connection layers 41, 42 establish the two diodes 31, 32 connected in 
parallel with opposite polarities between the pixel electrode 10 and scan 
electrode 20. 
The display panel of conventional type formed as described above often 
suffers from disconnection on the substrate. The disconnection is 
attributable to the loss of the ITO layer at locations indicated by 
hatching, P and Q, in FIG. 3. P is between the connection layer 41 and the 
pixel electrode 10, and Q is between the connection layer 42 and the scan 
electrode 20. This defect occurs when the aluminum connection layer is 
formed by selective etching. This etching is achieved with phosphoric acid 
or hydrofluoric acid. This etching solution should not corrode the metal 
oxide layer, such as an ITO layer. In reality, however, corrosion 
sometimes does occur. To prevent the propagation of corrosion to other 
parts of the pixel electrode and scan electrode, a protective layer is 
formed under the aluminum layer. This protective layer is a Cr layer or Ti 
layer, about 1000 .ANG. thick, formed by sputtering. The aluminum layer, 
which is 5000-6000 .ANG. thick, is formed on the protective layer. The 
unnecessary part of the aluminum layer is removed by etching with 
phosphoric acid or hydrofluoric acid, and the unnecessary part of the Cr 
layer of Ti layer remaining after etching is removed by etching with a 
mixed aqueous solution of ceric ammonium nitrate and perchloric acid. The 
protective layer greatly reduces the occurrence of disconnection. A 
disadvantage of using the protective layer is that two additional steps 
are required for the deposition and etching of the metal layer, leading to 
an increased production cost. It may be possible to use the Cr layer or Ti 
layer alone as the connection layer, eliminating the aluminum layer. This 
alternative, however, has another disadvantage; that is, comparatively 
brittle Cr or Ti under thermal stress easily breaks at the step (about 2 
.mu.m) between the diode and the pixel electrode or scan electrode as 
shown in FIGS. 3(b) and 3(c). 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a active matrix display 
panel that is substantially free of defects caused by disconnection. 
According to the present invention, this object is achieved by expanding 
the size of the insulating layer over that known in the art and forming at 
least one additional window in the insulating layer through which a 
portion of the pixel layer is exposed. Thus, a display panel according to 
the invention comprises a substrate having disposed thereon a plurality of 
display elements, each display element comprising 
(a) a pixel electrode disposed on the substrate; 
(b) a scan electrode disposed on the substrate separate from the pixel 
electrode; 
(c) display drive elements, for example a pair of photodiodes, one disposed 
over a first part of the pixel electrode and the other disposed over a 
first part of the scan electrodes; 
(d) an insulating layer extending over at least a second part of the pixel 
electrode and the display drive elements and having at least three 
openings formed therein, two of the openings being coincident with and 
exposing a portion of each of the display drive elements and a third 
opening being disposed so as to expose a portion of the second part of the 
pixel electrode; 
(e) a first connection layer disposed over the insulating layer and forming 
an electrical connection through the windows in the insulating layer 
between the exposed portion of the display drive element disposed over the 
scan electrode and the exposed portion of the pixel electrode; and 
(f) a second connecting layer disposed over the insulating layer and 
forming an electrical connection between the portion of the display drive 
element disposed over the pixel electrode exposed through the opening in 
the insulating layer and an exposed part of the scan electrode. 
Advantageously, a fourth window can be formed in the insulating layer so as 
to expose a part of the scan electrode, and the second connection layer 
will make electrical contact with this part of the scan electrode. 
The apparatus of the present invention as mentioned above produces the 
following effects: There is only a small possibility of disconnection 
occurring at the step even when a metal such as aluminum having a high 
ionization tendency is used for the connection layer. The metal oxide 
layer used as the pixel electrode and the like is not corroded at the time 
of etching. Thus, the probability that defects such as disconnection occur 
in the display drive elements is reduced by a great extent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be described in more detail with reference to the 
preferred embodiments illustrated in FIGS. 1 and 2. The reference 
characters used in FIG. 3 designate like or corresponding parts in FIGS. 1 
and 2. The description for overlapping parts is omitted for brevity. In 
the examples the display panel is a liquid crystal display, and the 
display drive element is a pair of amorphous silicon diodes connected in 
parallel with opposite polarities, the pixel electrode and scan electrode 
are made of ITO film, the insulation layer is silicon nitride, and the 
metal for the connection layer is aluminum. 
FIG. 1(a) is a plan view of a display element of a display panel in 
accordance with the invention. FIGS. 1(b), 1(c) and 1(d) are sectional 
views taken along lines B--B, C--C, and D--D, respectively, in FIG. 1(a). 
In accordance with the invention, insulation layer 60 has a larger area 
than that shown in FIG. 3, covering not only the two diodes 31, 32, but 
also a part of the pixel electrode 10 and the scan electrode 20. 
Additionally, the insulation layer may be square in shape. The insulation 
layer 60 is provided with two windows 60a at the center of the top of each 
of the diodes 31, 32, as in the case of the conventional product. 
Connection layer 41 for the diode 31 is connected through the window 60a 
to the light-shielding film 30a, which is the upper electrode of the diode 
31. The connection layer 41 is in conductive contact with the pixel 
electrode 10 through another window 60b formed in the insulation layer 60. 
The overlapping area 51 of the connection layer 41 and pixel 10 is 
indicated by hatching in the figure. The C-shaped peripheral part 51a of 
the connection layer 41 is the part in which the conventional ITO film, as 
the pixel electrode 10, is subject to corrosion. According to this 
invention, the insulation layer 60 is inserted under the periphery 51a and 
its edge is preferably at least 10 .mu.m away from the periphery 51a. 
In FIG. 1 the connection layer 42 for the diode 32 has a shape different 
from connection layer 41. As shown, connection layer 42 is L-shaped and a 
part of the connection layer makes conductive contact with the scan 
electrode 20 at a location outside the lower periphery of the insulation 
layer 60. The connection layer 42 is disposed over the scan electrode 20 
at the part 42a which is in conductive contact with the scan electrode 20, 
as shown in FIG. 1(d), and extends downward in the figure to the vicinity 
of the diode 31 for the pixel electrode. For convenience, the lower 
portion of the connection layer 42 of an adjacent display element is shown 
in the upper part of FIG. 1(a). The overlapping region 52 of the 
connection layer 42 and scan electrode 20 is indicated by hatching. The 
periphery of the connection layer 42 and the periphery of the overlapping 
region 52 is indicated by 52a, 52b. Under each of the peripheries 52a and 
52b, there is the insulation layer 60 for the scan electrode 20. As 
mentioned above, the scan electrode 20 is mostly covered with the 
connection layer 42 so that the resistance of the scan electrode is 
reduced as much as possible and the scan electrode is protected from 
disconnection and other defects. This structure permits the considerable 
reduction of the width of the scan electrode 20. 
Another embodiment of the invention is illustrated in FIG. 2. There are 
shown the diode pairs 31, 32 and 33, 34 for the adjacent two pixel 
electrodes 10, 11. These four diodes are arranged close to one another so 
that the area which they occupy is minimized. In this example, the 
insulation layer 60 is made of polyimide resin. It is used as a 
photoersist and can be thinner than 1000 .ANG. on account of its superior 
mechanical strength. The insulation layer 60 serves as both a continuous 
insulation layer and an alignment layer for the pixel electrodes 10, 11. 
It is deposited and etched such that it entirely covers not only the four 
diodes but also the two pixel electrodes 10, 11. The window 60a is formed 
in the insulation layer 60 at the top of each diode. It is through this 
window that one end of the respective connection layers 41 to 44 is 
connected to the upper electrode of the diodes 31 to 34. The other end of 
the connection layer is conductively connected to the pixel electrodes 10, 
11 and scan electrode 20 through the other window 60b formed in the 
insulation layer 60. That part of the insulation layer 60 on the pixel 
electrodes 10, 11 which is used as the alignment layer on which the diode 
or connection layer is not formed, undergoes rubbing after complete 
curing, so that the liquid crystal molecules in contact with the alignment 
layer are arranged in the specified direction. 
The pixel electrodes are advantageously made of a metal oxide layer because 
they are required to be transparent and electrically conductive for image 
display. It is not always necessary for the scan electrodes to be made of 
a metal oxide layer. The display drive elements, e.g., the photodiodes, 
are connected via a connection layer to either a pixel electrode and the 
scan electrode. These connections are protected from etch-induced defects, 
however, by the incorporation of an insulation layer interposed between 
the connection layer and the electrode at the point where the connection 
layer comes into contact with the electrode. This region, where the 
insulation layer is interposed, is also referred to as the overlapping 
region, as it is the point at which the connection layer and the 
electrodes overlap for their mutual connection. The insulation layer is 
provided to cover the display elements, for example a pair of photodiodes, 
for their protection, and may be extended to or beyond the overlapping 
region to cover remaining portions of the electrodes, referred to as the 
periphery. Alternatively, the insulation layer may be formed separately 
for the periphery. 
The metal oxide layer (such as ITO or tin oxide) used for the pixel 
electrode should not be easily corroded by an etching solution (such as 
phosphoric acid and hydrofluoric acid) for aluminum, or for a similar 
metal having a high ionization tendency conventionally used as the 
connection layer. This is confirmed by the fact that the metal oxide layer 
is not corroded when the aluminum layer on the flat part (central part) of 
the pixel electrode is removed by etching with the acid etching solution. 
The condition of etching the parts P and Q shown in FIG. 3 is different 
from the condition of etching the flat aluminum layer in that the solid 
aluminum layer to be left unetched is in direct contact with the metal 
oxide layer. In other words, in the case of etching on the flat part, the 
aluminum layer on the metal oxide layer is removed by etching sequentially 
from its surface, and when etching finally reaches the surface of the 
metal oxide layer, there exists almost no solid aluminum on the surface of 
the metal oxide layer. By contrast, in the case of etching at P and Q, 
there exists solid aluminum even when etching reaches the surface of the 
metal oxide layer. This solid aluminum in combination with the 
electrically conductive metal oxide layer form a kind of cell by the acid 
of the etching solution, which is an electrolyte. As a result, corrosion 
of the metal oxide layer takes place. As the metal (such as aluminum 
having a high ionization tendency) for the connection layer is dissolved 
in the acid etching solution, hydrogen is evolved, and this hydrogen 
reduces the metal oxide, causing the metal component to be dissolved in 
the etching solution. 
The present inventors completed this invention by paying attention to the 
above mentioned points. According to the present invention, the 
above-mentioned voltaic action is reduced to a great extent by interposing 
an insulation layer between the connection layer made of aluminum, or a 
similar metal having a high ionization tendency, and the metal oxide layer 
forming the pixel electrodes and the like. Even though the insulation 
layer is interposed, the metal of the connection layer is connected to the 
metal oxide layer of the electrode through the electrolytic etching 
solution which is in contact with the surface of the insulation layer. 
Therefore, theoretically, the possibility of voltaic action still exists. 
Nevertheless, the presence of the insulation layer weakens the voltaic 
action as compared with the conventional case, in which the metal is in 
direct contact with the metal oxide layer. The results of experiments 
indicate that the corrosion of the metal oxide layer can be substantially 
eliminated if a space of 10 to 20 .mu.m (depending on the kind of etching 
solution used) is provided by the surface of the insulation layer. 
In summary, the present invention provides an insulation layer interposed 
between the metal of the connection layer and the metal oxide layer of the 
pixel electrode or scan electrode. This structure prevents the metal oxide 
from corrosion by an etching solution. Without the insulation layer, 
corrosion takes place when the connection layer undergoes etching by 
photolithography on account of the strong voltaic action between the metal 
and the metal oxidelayer, which are in direct contact with each other. The 
construction in accordance with the invention, on the other hand, leads to 
a great decrease in the probability that defects by disconnection occur in 
display drive elements. The corrosion preventative effect is also produced 
in the case where a metal having a considerably high ionization tendency 
is used for the connection layer. Therefore, the present invention 
produces the following effects: Aluminum, which has been commonly used for 
connection, can be used alone for the connection layer. The connection 
layer for display drive elements has a high reliability. The coating and 
etching can be accomplished in one step each. According to the present 
invention, it is possible to simplify the process for producing the 
display panel of active matrix type and also to reduce the probability 
that defects occur in the display drive elements. The present invention is 
expected to contribute to the further development and commercialization of 
the display panel of large size and high display density.