Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-07-25
2004-12-28
Flynn, Nathan J. (Department: 2826)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S147000, C257S072000, C257S059000
Reexamination Certificate
active
06836299
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a thin film transistor liquid crystal display (TFT LCD) device, and more particularly to a reflective or transflective TFT LCD device having multi-layered pixel electrodes connected to drain electrodes with interposing an insulating layer therebetween.
BACKGROUND OF THE INVENTION
TFT LCD devices are generally classified into a reflective TFT LCD device using a reflective layer as pixel electrodes, a transmissive TFT LCD device using transparent pixel electrodes, and a transflective TFT LCD device using a reflective layer having a transmissive region in a portion of a reflective region as pixel electrodes, according to reflectance or permeability of pixel electrodes. In the TFT LCD devices, to supply voltage for controlling arrangement of liquid crystal to the pixel electrodes, drain electrodes of thin film transistors formed in each pixel are connected to the corresponding pixel electrodes. The pixel electrodes are generally connected to the drain electrodes through via holes formed in an interlayer insulating layer.
In a transmissive TFT LCD device, pixel electrodes use indium oxides to form transparent electrodes. However, this material may cause a problem that oxidizes wires of aluminum (Al) to form insulating oxides and thereby hinders in supplying voltage to the pixel electrodes. Therefore, in the transmissive TFT LCD device, drain electrodes are formed of a single layer of metal such as chromium (Cr), or a two-layered conductive layer having an Al-contained metal layer and a molybdenum tungsten (MoW) or Cr layer formed on the Al-contained metal layer.
In a reflective TFT LCD device, pixel electrodes usually use aluminum neodymium (AlNd). In this case, materials forming drain electrodes are also limited. Referring to
FIG. 1
, a pixel portion of the reflective TFT LCD device using an easily oxidized metal such as Al as source and drain electrodes
21
,
21
′ is illustrated. On the source and drain electrodes
21
,
21
′, a protecting layer
23
, for example a photo-sensitive organic insulating layer is disposed. The protecting layer
23
has via holes for connecting the drain electrodes
21
′ to the pixel electrodes
27
. Therefore, in an exposure, development and cleaning process of photolithography for forming the via holes, if developer or detergent of strong oxidant contacts the drain electrodes
21
′ through the via holes, the upper layer of the drain electrodes
21
′ may form insulating oxides
25
. The insulating oxides
25
increase contact resistance between the drain electrodes
21
′ and pixel electrodes
27
.
To solve the problem, an upper layer
212
′ of the drain electrode
21
′ can be formed of metal such as MoW that is resistant against oxidation, as shown in FIG.
2
. However, in this case, battery effect, like inside a chemical battery, can be occurred due to difference of electro-negativity between the upper layer
212
′ of the drain electrode
21
′ and an Al-containing reflective layer forming the pixel electrodes
27
. For example, due to corroding by the battery effect, gaps
29
similar to spike phenomenon generating at the interface between a silicon layer and an Al layer can be formed at the interface between the upper layer
212
′ and the Al-containing reflective layer. Also, as a portion of the Al-containing reflective layer around the gaps
29
falls, the Al-containing reflective layer can generate cracks
31
around the via holes. These gaps
29
or cracks
31
cause a problem increasing contact resistance between the pixel electrodes
27
and the drain electrodes
21
′.
Generally, the battery effect increases in proportion to the difference of surface area and electronegativity between two metal layers. Accordingly, the drain electrodes
21
′ that usually has relatively very small surface area compared to the pixel electrodes
27
enforces the battery effect more, thereby increasing the contact resistance between the pixel electrodes
27
and the drain electrodes
21
′ more.
To solve the battery effect, it can be considered to use a MoW or Cr layer as reflective plates or pixel electrodes
27
. However, such a choice deteriorates reflectance and conductivity of the pixel electrodes.
Accordingly, a new TFT LCD device, which can prevent increase of contact resistance due to the battery effect or surface oxidation at the interface between the pixel electrodes and the drain electrodes with maintaining highly reflectance and conductivity, is required.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved TFT LCD device that can prevent battery effect at the interface between pixel electrodes and drain electrodes, while maintaining high reflectance and conductivity.
It is another object of the present invention to provide an improved TFT LCD device that can prevent insulating oxides at the interface between pixel electrodes and drain electrodes, while maintaining high reflectance and conductivity.
It is other object of the present invention to provide an improved TFT LCD device that can prevent contact resistance increase at the interface between pixel electrodes and drain electrodes, while maintaining high reflectance and conductivity.
These and other objects are provided, according to the present invention, by a TFT LCD device having pixel electrodes formed of a multi-layered conductive layer. Preferably, drain electrodes are composed of multiple layers, and the most upper layer of the multiple layers is composed of a metal layer that is strongly resistant against oxidation. Also, the multi-layered conductive layer is composed of two-layered conductive layer having a lower layer of metal that has small electro-negativity difference between itself and the most upper layer of the drain electrodes and an upper layer of Al-containing metal.
In the present invention, the lower layer of the two-layered conductive layer is preferably composed of the same material as that of the most upper layer of the drain electrodes, for example one selected from a Cr layer and a MoW layer. The Al-containing metal usually uses pure Al or AlNd. Accordingly, the two-layered conductive layer is formed by depositing the lower layer of one selected from a Cr layer and a MoW layer and the upper layer of Al-containing metal and then patterning them.
It is noted that the multi-layered conductive layer is not limited to the two-layered conductive layer. To reduce the battery effect efficiently, if necessary, an intermediate metal layer can be interposed between the lower and upper layers of the two-layered conductive layer.
The multiple layers of the drain electrodes usually use metal having a high conductivity to prevent a signal voltage drop due to the data line resistance. Also, the drain electrodes are formed of the same conductive material as that of the data lines connected to source electrodes. Accordingly, the multiple layers forming the drain electrodes are preferably composed of a two-layered layer having an Al layer for increasing conductivity and a Cr or MoW layer strongly resistant to oxidation formed on the Al layer, or a three-layered layer having an intermediate Al layer and an upper and a lower Cr or MoW layers formed on and under the intermediate Al-contained layer to prevent spike phenomenon due to silicon layers over an active area. When the drain electrodes formed along with the data lines are of the three-layered layer, the MoW layer as the lower and upper layer preferably is better than the Cr layer since it is easy to be patterned along with the intermediate Al-containing layer.
According to the present invention, since the upper layer of the drain electrodes and the lower layer of the pixel electrodes are formed of same material or metals having small differences in electro-negativity, the battery effect therebetween can be ignored. Also, in the two-layered conductive layer of the pixel electrodes, the upper layer and the lower layer are concurrently formed to have the same surfac
Chung Woo-Suk
Hwang Chang-Won
Flynn Nathan J.
McGuireWoods LLP
Sefer Ahmed N.
LandOfFree
TFT LCD device having multi-layered pixel electrodes does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with TFT LCD device having multi-layered pixel electrodes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and TFT LCD device having multi-layered pixel electrodes will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3331347