1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device using thin-film transistors.
2. Description of the Related Art
In recent active matrix liquid crystal display devices using thin-film transistors (hereinafter abbreviated as TFTs), large screen sizes such as 20 inches or more and high resolution for SXGA (super extended graphics array) have been realized.
On the other hand, since active matrix liquid crystal display devices are applied to portable information equipment such as a notebook-sized personal computer and a PDA (personal digital assistant), attempts to reduce the power consumption are now being made.
One method for reducing the power consumption is to increase the aperture ratio. Ideally, it is intended to realize an aperture ratio of 80% or more for liquid crystal display devices of about 10 inches and an aperture ratio of 60% or more for those of about 4 inches.
One factor of reducing the aperture ratio is a storage capacitor in a pixel portion. In particular, in high-resolution panels such as small-size panels of 3 inches or less for use in a projector light bulb, the aperture ratio is reduced remarkably by the occupation area of a storage capacitor because the pixel area is small.
As for the classification of the structure of the storage capacitor, it may be formed by:
1) using a gate insulating film; PA1 2) using, an interlayer insulating film; PA1 3) using a passivation insulating film; and PA1 4) combining the methods of items 1)-3). PA1 a) a common electrode type using a dedicated capacitance wiring; and PA1 b) a combined use type using an adjacent scanning line. As a matter of fact, the connection method of item b) has a problem that if the scanning direction is changed, the image quality is lowered by a high voltage that instantaneously occurs at the pixel TFT side electrode. PA1 d) increasing the electrode area; PA1 e) decreasing the distance between the pixel electrode and the black matrix; and PA1 f) increasing the permittivity of the substance between the electrodes.
The structure of item 1) is widely employed because it can provide a large capacitance per unit area.
The connection method of the storage capacitor is classified into:
The present inventor has already filed a patent application relating to a scheme (top capacitor scheme) in which a storage capacitor is formed by a pixel electrode and a black matrix by forming the black matrix for light interruption on TFTs and using it as a common electrode, which scheme is an example of the common electrode type connection method using a dedicated capacitor line (Japanese Patent Application No.
Hei. 8-58500, which corresponds to U.S. patent application Ser. No. 08/646,512 filed on May 7, 1996).
This black matrix also has a function of shielding underlying bus lines such as source lines and gate lines. FIGS. 8A and 8B show a sectional structure of a pixel portion of a liquid crystal display device which employs the top capacitor scheme.
As shown in FIG. 8A, a double-gate TFT is formed on a substrate 801. The TFT has a semiconductor layer that consists of channel regions 802a and 802b, a source region 803, a drain region 804, and an impurity region 805. Gate electrodes 820a and 820b are formed on the semiconductor layer via a gate insulating film 810.
A first interlayer insulating film 811 made of an inorganic material is formed so as to cover the TFT. A source electrode 821 and a drain electrode 822 are in contact with the source region 803 and the drain region 804 of the TFT through contact holes, respectively.
A second interlayer insulating film 812 made of an organic resin is formed so as to cover the first interlayer insulating film 811 made of an inorganic material and the source electrode 821. The organic resin film 812 is provided so as to eliminate asperities due to the wiring of the TFT and other members, and its surface is approximately flat.
A conductive black film 824 is formed on the thus-formed organic resin film 812, and a third interlayer insulating film 813 made of an organic material is formed thereon. A pixel electrode 829 is in contact with the drain electrode 822 through a contact hole that is formed through the third interlayer insulating film 813 and the second interlayer insulating film 812, whereby the pixel electrode 829 is electrically connected to the drain region 804 of the TFT.
In FIG. 8A, a region indicated by character Y corresponds to a storage capacitor having the black matrix 824 and the pixel electrode 829 as its electrodes. The black matrix 824 is made of a metal such as Ti, Cr, or TiN.
FIG. 8B shows an example in which a contact hole is formed through the second interlayer insulating film, that is, the organic resin film 812 before formation of a black matrix 824 and a second drain electrode 825 is formed so as to contact the drain electrode 822 in forming the black matrix 824.
After a third interlayer insulating film 813 made of an organic material is formed, a pixel electrode 829 is brought into contact with the second drain electrode 825, whereby the pixel electrode 829 is electrically connected to the drain region 804 of the TFT.
Also in FIG. 8B, region Y corresponds to a storage capacitor using the black matrix 824 and the pixel electrode 829.
The top capacitor type storage capacitor (region Y) in the liquid crystal display device is composed of the black matrix, the pixel electrode, and the third interlayer insulating film made of an organic resin.
A conductive layer that is interposed between top and bottom organic resin layers in the above manner is prone to be affected by particles etc. in the adjacent organic resin layers. In particular, in many cases the conductive layer is used as an electrode and particles tend to cause short-circuiting in such a case.
In a study of the inventor, although particles whose sizes are larger than 0.5 .mu.m can be removed by filtering an organic resin before its use, it is difficult to completely remove particles whose sizes are smaller than 0.5 .mu.m.
Particles smaller than 0.5 .mu.m also remain in an organic material of the third interlayer insulating film that constitutes the storage capacitor. Therefore, short-circuiting may occur between the pixel electrode and the black matrix, to impair the capacitor function. Further, the short-circuiting may cause a display failure such as a point defect or a line defect.
According to the experience of the inventor, the probability of occurrence of a display failure due to a particle smaller than 0.5 .mu.m would be about one-hundred thousandth to one-millionth. For example, in a VGA (video graphics array) panel which has about three hundred thousand pixels, at most several point defects are caused by particles smaller than 0.5 .mu.m.
The capacitance C of the storage capacitor is expressed by the following equation: EQU C=.epsilon.S/d
where .epsilon. is the permittivity of the substance between the electrode, S is the electrode area, and d is the distance between the electrode.
Therefore, the capacitance of the storage capacitor can be increased by:
However, although various improvements have been made along the line of item d), basically the method of item d) decreases the aperture ratio and is difficult to implement particularly in small-size panels.
Although the method of item e) is easy to realize, thinning the third interlayer insulating film will surely increase the probability of occurrence of display failures due to particles because the third interlayer insulating film is made of an organic resin.
To realize the method of item f), it is necessary to dispose a substance having a large permittivity value (for instance, a silicon nitride film) between the electrodes. However, conventionally, to eliminate influences of asperities that are produced by electrodes, wiring lines, etc. when the black matrix is formed on TFTs, planarization is performed by using an organic resin film as the second interlayer insulating film.
It has been found that it is difficult to obtain a silicon nitride film of good quality by the current film forming methods, because gases of water, methane, etc. are generated from an exposed surface of the organic resin film as the second interlayer insulating film when a silicon nitride film is formed on the black matrix.
A silicon nitride film that is directly formed on the organic resin film is prone to peel off due to stress, which lowers the reliability of the liquid crystal display device.
Further, as shown in FIG. 9, where a silicon nitride film 918 is provided, to electrically connect a pixel electrode to a drain region 904, a third interlayer insulating film 913 is etched in a tapered manner and subsequently the silicon nitride film 918 is etched. In a subsequent step of etching a second interlayer insulating film 912, the etching goes around the ends of the silicon nitride film 918 to reach portions under the silicon nitride film 918.
This is because the etching rates of the second interlayer insulating film 912 and the third interlayer insulating film 913 are high while the etching rate of the inorganic layer 918 is low. This causes a problem that a pixel electrode contact failure is prone to occur at step portions indicated by character Z in FIG. 9.
Although contact holes are formed in a tapered manner to some extent, for the sake of simplicity the contact holes are shown by rectangles in the figures other than FIG. 9.