Optical: systems and elements – Polarization without modulation – Polarization by reflection or refraction
Reexamination Certificate
2001-06-06
2004-04-06
Shafer, Ricky D. (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization by reflection or refraction
C359S487030, C359S584000, C359S884000, C362S019000, C355S067000, C355S071000
Reexamination Certificate
active
06717729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to polarized light irradiation apparatus for optical alignment of liquid crystals in which an alignment layer of a liquid crystal display element is irradiated with polarized light.
2. Description of Related Art
Liquid crystal display elements are produced by carrying out an alignment process to align the liquid crystals in the desired direction on an alignment layer formed on the surface of a transparent substrate; two of these transparent substrates are put together with the alignment layers facing inwardly, such that alignment layers are separated by a gap of a desired size, and thereafter, liquid crystal is injected into the gap.
The alignment process for the alignment layer is a technology called optical alignment, in which the alignment layer is exposed by irradiation with polarized light of a particular wavelength.
The most widely used liquid crystal display elements, called TN liquid crystal, are constructed such that the direction of alignment of the liquid crystal is rotated 90° between the two transparent substrates. Accordingly, it is necessary to have two transparent substrates with their alignment layers in different directions.
Moreover, the angle of view of the liquid crystal can be improved by dividing one pixel of the liquid crystal display element into two or more parts and changing the alignment direction of the liquid crystal in each of the divided pixels. This method is called the pixel division method or the multi-domain method.
When optical alignment is applied to this pixel division method, a mask is used and one portion of the divided pixel formed on the substrate is irradiated with polarized light, after which the mask is replaced and the other divided portion is irradiated with light with a different polarization direction.
It is desirable that the polarized light irradiation apparatus, used to irradiate the alignment layer with polarized light, be provided with the ability to change the desired direction of polarization of the polarized light to any desired direction for irradiation. The present applicant has proposed, in EP 1020739 A2, a light irradiation apparatus in which the direction of the polarized light used to irradiate the substrate can be changed without rotating the substrate by means of a rotating the polarization element within the light irradiation apparatus. An example of the structure of the light irradiation apparatus capable of changing the direction of the polarized light is illustrated in FIG.
11
.
In
FIG. 11
, the light from the lamp
1
is condensed by an ellipsoidal condenser mirror
2
, reflected by the first plane mirror
3
and is incident on the polarization element
8
. The polarization element
8
is, for example, a device with multiple glass plates inclined at Brewster's angle with respect to the optical axis; light of polarization P passes through but most light of polarization S is reflected. By this method, it is possible to obtain polarized light with the desired extinction ratio. The polarized light P that emerges from the polarization element
8
is incident on the integrator lens
4
, then passes through the shutter
5
and is reflected by the second plane mirror
6
into the collimator lens
7
. The resulting parallel light rays pass through the mask M and irradiate the alignment layer of the workpiece (substrate) W. In this apparatus, the polarization element
8
is free to rotate around the optical axis of the center of the light flux that is incident on the polarization element
8
, and by rotating the polarization element
8
, it is possible to change and set, as desired, the direction of the polarized light that irradiates the alignment layer.
In addition, there are apparatus that, instead of rotating the polarization element
8
, rotates the substrate stage (not illustrated) on which is mounted the workpiece W substrate on which the alignment layer is formed.
In the light irradiation apparatus, the required length of the optical path is determined from opti-metric issues including the area irradiated, the distribution of intensity, and the degree to which the rays irradiating the substrate are parallel. In order to make the light irradiation apparatus smaller while still maintaining the same length of optical path, the optical path is folded using the first plane mirror
3
and the second plane mirror
6
.
These plane mirrors are fabricated by the vapor deposition of aluminum or some other metal on a quartz plate. To prevent damage to the mirror, the surface of the mirror is often covered with a protection layer, normally from 10 nm to more than 100 nm in thickness. Examples of such protection layers are magnesium fluoride (MgF
2
), silicon dioxide (SiO
2
), or aluminum oxide (Al
2
O
3
).
In order to optically align the alignment layer, the polarized light must be at a certain wavelength and must have an extinction ratio at or above a certain level. These values are determined by the properties of the alignment layer. The extinction ratio is the ratio between polarization P component of the light and the polarization S component. An extinction ratio of 10:1 or better is desirable for aligning the alignment layer. And it is common to use ultraviolet light with a wavelength in the range of 250 nm to 350 nm. However, in the apparatus in
FIG. 11
, even if a nearly linearly polarized light (with a good extinction ratio, between 20:1 and 15:1, for example) is emitted from the polarization element, it sometimes happens that when the polarization element is rotated to change the direction of polarization of the polarized light irradiating the substrate, the extinction ratio drops (to 6:1, for example) and the desired extinction ratio cannot be obtained.
As an example, if the polarization element is glass set at Brewster's angle, the polarized light P is a linearly polarized light. Further, it is known that generally, when linearly polarized light is reflected by a mirror, the phase shift of the reflected light can cause elliptical polarization. It is this elliptical polarization that is the cause of the extinction ratio reduction.
A simple explanation of the reason for elliptical polarization is shown in
FIG. 12
, which is an illustration showing the reflected state when a linearly polarized light emerges from the polarization element and is incident on the mirror with an angle of incidence of 45°. The polarized light that emerges from the polarization element has an electrical field that is aligned up and down with respect to the plane of the paper (the direction of polarization is up and down in the plane of the paper).
In
FIG. 12
, the optical axis of the polarized light that emerges from the polarization element is in the plane A determined by the direction of the electrical field of the polarized light. On the other hand, the optical axis of the light that is incident on the mirror (which is the light that emerges from the polarization element) is in plane B determined by the optical axis of the light reflected from the mirror.
FIG. 12
illustrates the situation where plane A and plane B are parallel to each other. However, if the polarization element is rotated around the optical axis, plane A and plane B are no longer parallel, and the polarization element is inclined 90° from its state in
FIG. 12
; plane A and plane B would then be perpendicular to each other.
At the plane of reflection of the mirror, the component of the light that is incident on the mirror that is parallel to plane B is the polarization component P, and the component that is perpendicular to plane B is the polarization component S.
When plane A and plane B are in a parallel or perpendicular relationship, the direction of the electric field of the polarized light incident on the mirror (the direction of polarization) can have only a polarization P component or a polarization S component. For example, if the direction of the electric field of the polarized light incident on the mirror (the direction of polarization) is as shown in
FIG. 13
, th
Goto Manabu
Shinbori Masashi
Nixon & Peabody LLP
Safran David S.
Shafer Ricky D.
Ushiodenki Kabushiki Kaisha
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