Vertically aligned type liquid crystal display

Details Vertically aligned type liquid crystal display Vertically aligned type liquid crystal display

2001-01-23

2002-06-18

Sikes, William I. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S142000, C349S144000, C349S146000

Reexamination Certificate

active

06407794

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) which utilizes opto-electric anisotropy of liquid crystal, and more particularly to a liquid crystal display which achieves an improved response speed and transmittance.
2. Description of the Related Art
LCDs are compact, thin, and low power consumption devices and have been developed for practical use in the field of office automation (OA) and audio-visual (AV) equipment. In particular, active matrix type LCDs which utilize thin film transistors (TFTs) as switching elements are theoretically capable of static actuation at a duty ratio of 100% in a multiplexing manner, and have been used in large screen and high resolution type animation displays.
TFTs are field effect transistors arranged in a matrix on a substrate and connected to individual pixel electrodes which form one side of pixel capacitors with a dielectric layer made of liquid crystal. In a TFT matrix, TFTS located on a same row are simultaneously turned on/off by a given gate line, and each TFT of that row receives a pixel signal voltage from a given drain line. A display voltage is accumulated in the pixel capacitors corresponding to the on-state TFTs and designated by rows and columns. The pixel electrodes and the TFTs are formed on the same substrate, while a common electrode acting as the other side of the pixel capacitors is formed on the entire surface of the second substrate opposite to the first substrate across the liquid crystal layer. That is, the display pixels (i.e., pixels) are defined by partitioning the liquid crystal and the common electrode by pixel electrodes. The voltage accumulated in the pixel capacitors is held insulated by an off-state resistance of the TFTs for one field period or one frame period until the TFTs are turned on again. The liquid crystal is opto-electrically anisotropic, and its transmittance is controlled based on the voltage applied to respective pixel capacitors. The transmittance of each display pixel is independently controlled, so that individual pixels are observed bright or dark and recognized collectively as a display image by human eyes.
Initial orientation of the liquid crystal is determined by an orientation film disposed at the interface between the liquid crystal and each substrate. For example, a twisted nematic (TN) type LCD uses the liquid crystal in nematic phase which has positive dielectric anisotropy and whose alignment vectors are twisted 90 degrees between opposing substrates. Typically, a polarizing plate is provided on the outside of each substrate, and an polarizing axis of each polarizing plate coincides with the orientation of the liquid crystal located in the vicinity of the corresponding substrate. When no voltage is applied, linearly polarized light passes through one polarizing plate, turns its direction in the liquid crystal layer along the twisted alignment of the liquid crystal, and exits from the other polarizing plate, resulting in a “white” display. When the voltage is then applied to the pixel capacitors, an electric field is created within the liquid crystal and the orientation of the liquid crystal is changed to be parallel to the direction of the applied electric field because of dielectric anisotropy. This results in the collapse of twisted alignment and less frequent turns of the linearly polarized incoming light in the liquid crystal. Consequently, the amount of light ejecting from the other polarizing plate is reduced and the display gradually becomes black. This is known as a normally white mode which is widely applied in the field of TN cells, in which the display is white when no voltage is applied and changes to “black” upon application of the voltage.
FIGS. 1 and 2
show a unit pixel structure of a conventional liquid crystal display, wherein
FIG. 1
is a plan view and
FIG. 2
is a sectional view along line G—G of
FIG. 1. A
gate electrode
101
made of a metal, such as Cr, Ta, or Mo, is formed on a substrate
100
, and a gate insulating film
102
made of, e.g., SiNx and/or SiO
2
is formed to cover the gate electrode
101
. The gate insulating film
102
is covered with a p-Si film
103
in which an implantation stopper
104
is used to form a lightly doped region (LD) having a low concentration (N−) of impurities, such as P or As, and source and drain regions (S, D) having a high concentration (N+) of impurities located outside the LD region. A region located immediately below the implantation stopper
104
is an intrinsic layer which includes substantially no impurities and acts as a channel region (CH). The p-Si
103
is covered with an interlayer insulating film
105
made of SiNx or the like. A source electrode
106
and a drain electrode
107
, both made of a material such as Al, Mo, or the like, are formed on the interlayer insulating film
105
, each electrode being connected to the source region S and the drain region D, respectively, via a contact hole CT
1
formed in the interlayer insulating film
105
. The entire surface of the thus formed TFT is covered with a planarization insulating film
108
made of SOG (spin on glass), BPSG (boro-phospho silicate glass), acrylic resin, or the like. A pixel electrode
109
made of ITO (indium tin oxide) or the like is formed on the planarization insulating film
108
for actuating the liquid crystal, and is connected to the source electrode
106
via a contact hole CT
2
formed in the planarization insulating film
108
.
An orientation film
120
formed by a high molecular film, such as polyimide, is disposed on the entire surface on the above elements and undergoes a rubbing treatment to control an initial orientation of the liquid crystal. Meanwhile, a common electrode
131
made of ITO is formed on the entire surface of another glass substrate
130
arranged opposite to the substrate
100
across a liquid crystal layer. The common electrode
131
is covered with an orientation film
133
made of polyimide or the like and undergone rubbing.
As shown herein, a DAP (deformation of vertically aligned phase) type LCD uses a nematic phase liquid crystal
140
having negative dielectric anisotropy, and orientation films
120
,
133
formed by a vertical orientation film. The DAP type LCD is one of the electrically controlled birefringence (ECB) type LCDs which use a difference of refractive indices of longer and shorter axes of a liquid crystal molecule, so-called a birefringence, to control transmittance. In the DAP type LCD, upon application of a voltage, an incoming light transmits one of two orthogonal polarization plates and enters the liquid crystal layer as a linearly polarized light, and is birefracted in the liquid crystal to become an elliptically polarized light. Then, retardation, which is a difference of phase velocity between ordinary and extraordinary ray components in the liquid crystal, is controlled according to an intensity of the electric field of the liquid crystal layer to allow the light to be emitted from the other polarization plate at a desired transmittance. In this case, the display is in a normally black mode, since the display is black when no voltage is applied and changes to white upon application of an appropriate voltage.
As described above, the liquid crystal display displays an image at an intended transmittance or color phase by applying a desired voltage to the liquid crystal sealed between a pair of substrates having predetermined electrodes formed thereon and by controlling a turning route or a birefringence of light in the liquid crystal. Specifically, the retardation is controlled by changing the alignment of the liquid crystal, to thereby adjust the light intensity of the transmitted light in the TN mode, while allowing the separation of color phases in the ECB mode by controlling a spectroscopic intensity depending on wavelength. Since the retardation depends on the angle between the longer axis of the liquid crystal molecule and the orientation of the electric field, the retardation still changes relati

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