Patent Application
20060270243
This invention relates to an evaporator for depositing film with a uniform orientation to form an alignment layer for a liquid crystal display.
Thin film deposition refers to a process that coats a thin layer of film on a substrate. Thin film deposition includes sputtering, evaporation, and chemical vapor deposition. Thin film deposition is generally concerned with the thickness and the uniformity of the film, and not the alignment orientation of the thin film.
On a liquid crystal display (LCD), there are alignment layers above and below the liquid crystal that control the alignment of the liquid crystal molecules. The surfaces of the alignment layers have the proper geometric patterns to fix the liquid crystal molecules along the desired directions. An alignment layer can be polyimide that is spun onto a substrate and then mechanically rubbed to form microgrooves along the desired direction. The liquid crystal molecules mount onto the microgrooves along the desired direction. The alignment layer can also be silicon dioxide that is deposited onto a substrate at an oblique angle to form the microgrooves.
One of the most popular LCD display is the vertically aligned nematic (VAN) display. In the VAN display, the liquid crystal molecules are aligned almost perpendicular to the surface with a small pre-tilt angle at the off state. To improve respond speed and contrast, the alignment layers are formed with a uniform pre-tilt angle. Depending on the design, the pre-tilt angle ranges between 0.5 to 10 degrees.
In a conventional evaporator, an electron gun irradiates a target material in a crucible within a vacuum chamber. Once heated, the target material evaporates and the evaporated particles attach to a substrate suspended to form a thin film. However, the upward directions of the evaporated particles vary. This makes it difficult to form a VAN alignment layer having a uniform pre-tilt angle. It is especially difficult during a mass production process where many substrates hang inside the vacuum chamber. The alignment directions of the thin films on the substrates will differ according to each substrate's position and height, thereby adversely affecting the uniformity of the LCD displays.
Thus, what is needed is an apparatus to evaporate a film with uniform alignment orientation onto a substrate.
In one embodiment of the invention, a system for depositing film includes (1) a substrate holder assembly, (2) a particle source, (3) an alignment shield between the particle source and the substrate holder assembly, and (4) a vacuum chamber for enclosing the substrate holder assembly, the particle source, and the alignment shield. The alignment shield includes alignment cells each having walls that can be individually adjusted to change the oblique deposition angles. By adjusting the walls of the alignment cells, uniform oblique deposition angles can be achieved on substrates held by the substrate holder assembly. The deposited film with uniform orientation can be used as the alignment layer of a liquid crystal display.
Use of the same reference numbers in different figures indicates similar or identical elements.
Particle source 104 (e.g., an evaporation source) includes a crucible 112 for holding a target material 114 (e.g., silicon dioxide), and an electron gun 116 for heating target material 114. Although only one evaporation source is shown, multiple evaporation sources can be used. Each evaporation source may employ its own electron gun or share a common electron gun where the crucibles are transported to the electron gun. Typically silicon dioxide is evaporated onto a silicon substrate or an indium tin oxide (ITO) glass wafer to form the alignment layer of a liquid crystal display (e.g., a VAN display).
The distance between each substrate 207 to evaporation source 104 may be different. Furthermore, the position of each substrate 207 relative to evaporation source 104 may be different. In addition, the distance between evaporation source 104 and substrate holder assembly 106 may be adjusted to achieve a desired deposition rate. All these factors affect the oblique deposition angle and the pre-tilt angle of the resulting alignment layer.
These factors can be easily addressed by alignment shield 108 with individually adjustable alignment cells 302. Each alignment cell 302 is tailored for one corresponding substrate. Each substrate may receive evaporated particles from a single alignment cell 302 or from a group of alignment cells 302. Accordingly, alignment layers with uniform pro-tilt angle can be formed on each of the substrates.
The dimensions of the columns and rows and the height of the walls depend on the actual dimensions of evaporator 100. In general, the dimensions of the columns and rows can be decreased and the height of the walls can be increased to limit the evaporated particle trajectories that pass through alignment cells 304, thereby reducing the variation in the oblique deposition angle. The distance between substrates 207 suspended by substrate holder assembly 106 and evaporation source 104 can be increased to limit the variation in the oblique deposition angle. The distance between alignment shield 108 and evaporation source 104 can be increased to limit the variation in the oblique deposition angle.
As described before, each alignment cell 304A is individually tailored for one corresponding substrate to compensate for the varying distances between that substrate and evaporation source 104 and for the position that substrate relative to evaporation source 104. Each substrate may receive evaporated particles from a single alignment cell 302A or from a group of alignment cells 302A. Accordingly, alignment layers with uniform pre-tilt angle can be formed on each of the substrates.
Evaporation source 104 includes crucible 112 for holding target material 114, and electron gun 116 for heating target material 114. Although only one evaporation source is shown, multiple evaporation sources can be used.
Substrate holder assembly 502 may have a dome shape with individual substrate holders 508 for holding the substrates. Alignment shield 504 has a dome shape similar to substrate holder 502 to provide a better correspondence between alignment cells 510 and the substrates. As described above with alignment cells 304, each alignment cell 510 has four adjustable walls 512 that can be adjusted to provide the desired oblique deposition angle.
Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. For example, alignment cells can have a hexagon shape with six walls. Furthermore, the alignment shield can have any shape that matches the shape of the substrate holder. Numerous embodiments are encompassed by the following claims.
