1. Field of the Invention
The present invention relates to a reflective or semi-transmissive liquid crystal display device for displaying a picture by utilizing outside light. In particular, the present invention relates to a liquid crystal display device in which fine bumps and dips are provided in the surface of a reflective film, and relates to a method of manufacturing the same.
2. Description of the Prior Art
Liquid crystal display devices have the advantages that they are thin and light and that they can be driven at low voltages and have low power consumption, and are widely used in various kinds of electronic devices. In particular, active matrix liquid crystal display devices, in which a thin film transistor (TFT) is provided as a switching element for each picture element, also show excellent display quality comparable to that of cathode-ray tube (CRT) displays, and are therefore widely used as displays for televisions, personal computers, and the like.
A typical liquid crystal display device has a structure in which liquid crystals are contained between two substrates placed to face each other. On one substrate, TFTs, picture element electrodes, and the like are formed; on the other substrate, color filters, a common electrode, and the like are formed. Hereinafter, the substrate on which the TFTs, the picture element electrodes, and the like are formed is referred to as a TFT substrate, and the substrate placed to face the TFT substrate is referred to as a counter substrate.
Liquid crystal display devices include transmissive liquid crystal display devices in which a backlight is used as a light source and in which a picture is displayed using light passing through a liquid crystal panel, reflective liquid crystal display devices in which a picture is displayed by utilizing the reflection of outside light (natural light or lamp light), and semi-transmissive liquid crystal display devices in which a picture is displayed using a backlight in a dark place or using the reflection of outside light in a well-lighted place.
The reflective liquid crystal displays have the advantage that power consumption is smaller than that of the transmissive liquid crystal display devices because a backlight is unnecessary. Further, in a place where the surrounding area is well lighted, a picture often can be seen well in a reflective liquid crystal display device or a semi-transmissive liquid crystal display device utilizing outside light, compared to a transmissive liquid crystal display device utilizing a backlight.
Incidentally, in reflective liquid crystal display devices and semi-transmissive liquid crystal display devices, if the surface of a film (reflective film) for reflecting light is smooth, a range (viewing angle) in which a picture can be seen well becomes extremely narrow, and the problems of glare and the like occur. Accordingly, it is necessary to scatter light by providing fine bumps and dips in the surface of the reflective film.
Heretofore, a method has been proposed in which fine bumps and dips are formed in the surface of a reflective film. For example, in Japanese Unexamined Patent Publication No. Hei 5(1993)-173158, a technology is described in which bumps and dips are formed in the surface of an organic insulating film (polyimide film) using photolithography and dry etching and in which a reflective film is formed thereon. Further, in the specification of Japanese Patent No. 2990046, a technology is described in which bumps and dips are formed by utilizing at least one of a metal film, an insulating film, and a semiconductor film used to form a switching element (TFT) and in which a reflective film is formed thereon with an insulating film interposed therebetween.
However, the inventors of the present application consider that the above-described known technologies have the problems described below. That is, the technology disclosed in Japanese Unexamined Patent Publication No. Hei 5(1993)-173158 requires the step of spreading photosensitive resin (photoresist) on the organic insulating film, the steps of exposure and development, and the step of dry etching. Accordingly, with an increase in the number of steps, manufacturing cost increases, and yield decreases.
In the technology described in the specification of Japanese Patent No. 2990046, a metal film, an insulating film, and a semiconductor film are deposited, these films are etched by photolithography, bumps and dips are formed simultaneously with TFTs, then an insulating film is formed on the entire surface, and further a reflective film is formed thereon. An increase in the number of manufacturing steps can be avoided by forming the bumps and dips simultaneously with the TFTs as described above. However, with this technology, it is difficult to form bumps and dips at high density because the density of bumps and dips depends on the resolution of photolithography.
Moreover, in part of embodiments described in the specification of Japanese patent No. 2990046, a glass substrate is etched. However, if a glass substrate is etched, impurities contained in the glass substrate are eluted to contaminate liquid crystals, and display quality may be significantly impaired.