1. Field of the Invention
The present invention relates in general to an active matrix substrate for a liquid crystal display (LCD) and method of forming the same. In particular, the present invention relates to a liquid crystal display having a high pixel aperture ratio (pixel opening size) and method of forming the same.
2. Description of the Related Art
Generally, LCDs have an upper and a lower substrate with electrodes thereon. These substrates are sealed with adhesive materials, and a liquid crystal material is sealed between these two substrates. Before the liquid crystal is injected between the two substrates, spacers are sprayed between the substrates in order to hold a constant distance therebetween. Conventionally, many TFTs are formed above the lower substrate as switching devices. Each TFT has a gate electrode connected to a scanning line, a drain electrode connected with a signal line, and a source electrode connected to a pixel electrode. The lower substrate is also called an active matrix substrate. The upper substrate includes a color filter and a common electrode.
The higher the pixel aperture ratio of a LCD, the higher the display transmission. Thus, by increasing the pixel aperture ratio of a LCD, transmission may be increased using the same backlight power, or alternatively, the backlight power consumption may be reduced while maintaining the same display transmission.
In order to enhance the pixel aperture ratio, a thicker insulating layer is formed over source and drain electrodes before forming pixel electrodes, thus, the pixel electrodes are formed over the insulating layer so as to overlap portions of the address lines, as disclosed in U.S. Pat. Nos. 5,955,744, 5,780,871, 5,641,974. Thus, the capacitance between pixel electrodes and underlying conducting material can be reduced, and an effective display area, i.e. the area of the pixel electrode, can be enlarged.
It is an object of the present invention to provide a method of forming an active matrix substrate for a liquid crystal display with a high pixel aperture ratio and method of forming the same.
According to one aspect of the invention, an active matrix substrate for a liquid crystal display is provided. A plurality of gate lines are disposed on a transparent substrate, parallel to a first direction and with protruding portions covering active device regions. A gate insulating layer is disposed on the gate lines and the transparent substrate. A low k dielectric layer is disposed on the gate insulating layer, and has first openings corresponding to the active device regions. A plurality of data lines are disposed between the low k dielectric layer and the gate insulating layer, perpendicular to the first direction and with protruding portions covering the active device regions. The data lines and the gate lines define regions including the active device regions and the pixel regions. A first n-doped layer is disposed under the data lines, contacts the data lines and has the same pattern as a gathering of the data lines, wherein the first n-doped layer at one side of the first openings providing sources. Conducting lines with source pattern are disposed at the first openings on the sources, with the protruding portions of the data lines contact source electrodes. Second n-doped layer is disposed under the data lines, and contacts the drain electrodes belonging to the protruding portions of the data lines, wherein the second n-doped layer at the other side of the first openings provides drains. A channel exists between each source and its corresponding drain. A semiconductor layer is disposed under the first n-doped layer and the second n-doped layer on the gate insulating layer and has the same pattern as a gathering of the first n-doped layer, the second n-doped layer and the channels. Pixel electrodes are disposed on the low k dielectric layer at the pixel regions and contact the drains. A passivating layer is disposed on the first openings.
According to another aspect of the invention, an active matrix substrate for a liquid crystal display can be formed by the following steps. Gate lines are formed on the transparent substrate, parallel to a first direction and with protruding portions covering a plurality of active device regions. A gate insulating layer is formed on the gate lines and the transparent substrate. A semiconductor layer is formed on the gate insulating layer. An n-doped layer is formed on the semiconductor layer. A metal layer is formed on the n-doped layer. The semiconductor layer, the n-doped layer and the metal layer are patterned and the metal layer is transferred to data lines perpendicular to the first direction. A low k dielectric layer is formed on the transparent substrate having the data lines thereon. First openings are formed in the low k dielectric layer corresponding to the active device regions. A transparent conducting layer is formed on the low k dielectric layer. The transparent conducting layer is patterned to pixel electrodes and conducting lines with source pattern. The data lines and the n-doped layer corresponding to the first openings are etched, so the data lines and the n-doped layer have the same local pattern as the pixel electrodes and the conducting lines in the first openings. A passivating layer is formed on the first openings.
To form the active matrix substrate five masks are needed. The first mask forms gate lines on the transparent substrate. The second mask patterns a stacked layer of a metal layer/an n-doped layer/a semiconductor layer formed on a gate insulating layer to form data lines. After forming a low k dielectric layer, the third mask forms openings therein. The forth mask patterns pixel electrodes and conducting lines with source pattern on the low k dielectric layer and further patterns the metal layer and the n-doped layer. After depositing a passivating layer the fifth mask defines the passivating layer.