Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-08
2002-06-18
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C438S030000
Reexamination Certificate
active
06407786
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device (LCD) permitting reflection-mode display and a method for fabricating such a liquid crystal display device.
In recent years, LCDs have been broadly used for a variety of apparatuses including word processors, personal computers, TV sets, video cameras, still cameras, monitors for cars, portable office automation (OA) appliances, and portable game machines.
The LCDs do not emit light themselves, unlike cathode ray tubes (CRTs) and electro luminescence (EL) devices. Therefore, in the case of a transmission LCD using pixel electrodes (transparent electrodes) made of a transparent conductive material such as indium tin oxide (ITO), an illuminator such as a fluorescent tube (so-called backlight) is disposed at the rear of a liquid crystal panel so as to effect display using light emitted from the illuminator. Such a transmission LCD exhibits higher display quality but dis-advantageously consumes larger power, compared with a reflection LCD described hereinafter. A backlight normally consumes 50% or more of the total power of the LCD.
In order to solve the above problem of the transmission LCD, there have recently been developed a reflection LCD including pixel electrodes (reflection electrodes) made of a material having a reflection characteristic such as a metal and a transmission/reflection combination type LCD including both a transparent electrode and a reflection electrode for each pixel.
The LCD permitting reflection-mode display as described above includes a reflector for reflecting ambient light. Such a reflector may be placed inside a pair of substrates constituting a liquid crystal panel (internal type) or outside the rear substrate (on the side of the substrate opposite to the side of a liquid crystal layer) (external type). The internal type is advantageous in being free from an occurrence of double image due to the thickness of the substrate (typically, glass substrate). In addition, since the internal type reflector is typically made of a metal exhibiting electrical conductivity such as aluminum, it can also be utilized as pixel electrodes (or part of pixel electrodes). This simplifies the construction.
In order to realize display with a good paper-white property in the reflection mode, the reflector should preferably have an appropriate diffuse reflection characteristic (light distribution). If the reflection plane is close to a mirror plane, it mostly returns specular reflection (mirror reflection), causing a trouble of reflecting ambient images in some cases. In reverse, if the diffuse reflection characteristic is too large, the brightness lowers. It is therefore preferable to adjust the diffuse reflection characteristic so that good paper white property and brightness can be obtained.
A method for forming an internal type reflector (or reflection electrodes) is disclosed in Japanese Laid-Open Patent Publication No. 9-292504 (corresponding U.S. Pat. No. 5,936,688) of which applicant is the same as the assignee of the present application. U.S. Pat. No. 5,936,688 is incorporated herein by reference. In the disclosed method, a photolithography process and a heat treatment process are combined as described below.
A photosensitive resin film formed on a substrate is exposed to light via a photomask having a predetermined pattern and developed, to form a convex/concave profile corresponding to the predetermined pattern. The photomask, for example, has circular light-shading spots randomly distributed therein if a positive photosensitive resin is used. The photosensitive resin film having the convex/concave profile is then heat-treated to smooth the convex/concave profile utilizing the thermal deformation of the resin. A metal film is then formed over the resultant smooth convex/concave (continuously waved) surface, and patterned to a predetermined shape corresponding to the shape of pixels thereby to form a reflector.
In the exposure of a photosensitive resin, an exposure system (i.e., aligner) such as a stepper or a large-scale one-shot (full plate) exposure system is normally used. A stepper is preferably used for forming a photosensitive resin film as described above, that is, the photosensitive resin film having the convex/concave surface that determines the surface profile of the reflector required to have an appropriate diffuse reflection characteristic. The reason is as follows. A large-scale one-shot exposure system allows a large area to be exposed to light at one time, but has large in-plane variations in light intensity and degree of collimation. This makes it difficult to obtain a reflector having a good diffuse reflection characteristic. The convex/concave profile of the surface of the photosensitive resin film substantially determines the surface profile of the reflector. Accordingly, if the convex/concave profile lacks uniformity over the entire surface, the diffuse reflection characteristic of the reflector varies, resulting in failure in uniform display. If a large-scale one-shot exposure system is employed, the resultant reflection characteristic will be such that only the center of an exposed area is bright while the periphery thereof are dark, for example. It is therefore difficult to obtain a reflector suitable for practical use.
In other words, precise profile control is required in order to form an underlying layer having a predetermined surface profile for controlling the surface profile of the reflector so that the reflector has an appropriate diffuse reflection characteristic. This is different from the case of forming contact holes that do not directly influence the display. If the exposure is made using light having large in-plane variations in intensity and degree of collimation, it is impossible to process the surface of the underlying layer into a predetermined profile.
In a stepper, light from a light source is nearly collimated via a lens system, whereby in-plane variations in light intensity and degree of collimation are made small. The stepper however has a disadvantage of exposing only a small area at one time. For example, as shown in
FIG. 23A
, a region
87
a stepper can expose at one time is only about 6 inches (about 152.4 mm) in diameter. This means that a region
86
allowed for formation of a convex/concave profile is a square of about 6 inches in diagonal at maximum. In order to expose an area larger than 6 inches in diameter using the stepper, division exposure as shown in
FIG. 23B
is required, where an area is divided into sub-regions for individual exposure. More specifically, a first sub-region
88
a
is exposed to light in the first exposure step (an exposure range
89
a
; an exposure center
90
a
), and thereafter a second sub-region
88
b
is exposed to light in the second exposure step (an exposure range
89
b
; an exposure center
90
b
).
However, the exposure using a stepper still exhibits an in-plane variation of light (image distortion; while the degree of collimation of a light ray is high in the center, it is low in the periphery). The resultant light pattern obtained using the stepper is distorted from an ideal pattern that would be obtained if a photomask is illuminated with completely collimated light (the pattern of light-transmitting spots of the photomask). The distortion is greater in a portion closer to the periphery. For example, if a mask for illuminating a plurality of circular spots is used, circles are obtained in the center of an exposed area but ellipses instead of circles are formed in the periphery. In the case of one-shot exposure, when a reflector is formed on the resultant convex/concave surface, the reflection characteristic changes as the convex/concave pattern changes from circles to ellipses from the center toward the periphery of the exposed area. However, this change in reflection characteristic is continuous and thus hardly recognized as a change in display quality.
In the case of division exposure, however, the following problem arises. When a reflector is formed on the convex/concave surfa
Hara Takeshi
Kobayashi Kazuki
Kubo Masumi
Yamamoto Akihiro
Nixon & Vanderhye P.C.
Parker Kenneth
Sharp Kabushiki Kaisha
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