1. Field of the Invention
The present invention relates to an optical component such as a polarizing beam splitter or a phase plate in which a multilayer film is provided on a surface of a substrate to polarize incident light, and in particular, to an optical component having a multilayer film with a high aspect ratio as well as a method of manufacturing the same.
2. Related Background Art
Hitherto, an optical component such as a lens, used in an electronic equipment is based on an optical interference and has thin layers stacked on a surface of an optical element as an antireflection film. Further, on the basis of a similar optical interference, thin layers are stacked on a surface of a substrate composed of a transparent optical element to polarize an incident light, thereby forming a polarizing beam splitter or a phase plate. Such an interference film is inexpensive and has good reproducibility, and is thus put to practical use widely.
FIG. 5 shows an example of a conventionally used interference film cubic type polarizing beam splitter. In FIG. 5, reference numerals 12a and 12b denote transparent body substrates each having a surface inclined at 45°. Reference numerals 13 and 14 denote a low refractive index layer and a high refractive index layer, respectively. Further, the arrows shown in the figure indicate the direction in which a light travels. A normal light has a S polarization beam that is a vibration component having a light vibrating direction perpendicular to the drawing of FIG. 5 and a P polarization beam that is a vibration component having a light vibrating direction parallel to the drawing of FIG. 5. The polarizing beam splitter can split an incident light into two components as shown in the figure by allowing the P polarization beam to pass therethrough while reflecting the S polarization beam.
However, although the cubic type polarizing beam splitter shown in FIG. 5 is very easy to manufacture and has good reproducibility, there is a problem as to its optical characteristics that the reflection and transmission characteristics change markedly depending on the incident angle of light.
For example, when the size of a device such as a liquid crystal projector is to be reduced, its optical path length must be shortened. The use of the conventionally used interference film cubic type polarizing beam splitter enables the optical path length to be shortened. However, because the angle at which a light is incident on the polarizing beam splitter varies markedly depending on the position of the incidence, the reflection or transmission characteristics are not constant, thus making it impossible to obtain uniform spectral characteristics within a plane. Therefore, to further improve the functions of electronic equipments, there is a need for an optical component such as a polarizing beam splitter or a phase plate that provides good optical characteristics, regardless of the incident angle of light.
To meet this need, a polarizer has recently been proposed which polarizes an incident light utilizing the anisotropy of propagation characteristics of the periodic structure of a transparent body and in which high refractive index layers and low refractive index layers are repeatedly stacked alternately on each other to provide a groove form that is bent periodically at a pitch less than the wavelength of visible light as disclosed in Japanese Patent Application Laid-Open No. 2000-56133. This publication describes the use of Si, GaAs, TiO2, and TaO2 as the high refractive index layer and the use of SiO2 as the low refractive index layer. Further, this publication describes an example in which the high refractive index and the low refractive index layers has a periodic thickness of 0.32 μm and in which the groove has a pitch of 0.4 μm and a depth of 0.2 μm.
With the configuration described in Japanese Patent Application Laid-Open No. 2000-56133 above, the polarizer can effectively function as a polarizing beam splitter for a light of a wavelength of 1 μm. However, the polarizer is difficult to use if an incident light to be polarized is a visible light having a wavelength of 400 to 700 nm. That is, Si and GaAs that constitute the high refractive index layer has a low transmissivity for visible light. Thus, the polarizer is difficult to use in an optical component such as a liquid crystal projector which must allow a light of a visible range to pass therethrough. Further, because the difference between the refractive index of TiO2 and TaO2 constituting the high refractive index layer and that of SiO2 constituting the low refractive index layer is too small, sufficient polarization characteristics cannot be obtained.
OPTICS LETTERS (Rong-Chung et al., Vol. 21, No. 10, p. 761, 1996) describes a polarizing beam splitter comprising a plurality of dielectric multilayer films arranged in lines on a transparent substrate as shown in FIG. 6. In FIG. 6, reference numeral 21 denotes an optical component such as a beam splitter or a polarizing plate. Reference numeral 22 denotes a transparent substrate as an optical element, and reference numerals 23 and 24 denote low refractive index and high refractive index layers, respectively. The low refractive index layers 23 and the high refractive index layers 24 are alternately stacked on a surface of the transparent substrate 22 to form an HL alternate layer 25. Reference numerals 26a, 26b, 26c, 26d denote dielectric multilayer films each formed in a line. It is known that the line-shaped dielectric multilayer films 26a, 26b, 26c, 26d which form periodic recesses and protrusions function as a diffraction grating and have excellent characteristics as an optical component that polarizes an incident light. Specifically, even if an incident light is a visible light having a wavelength of 400 to 700 nm, good polarization characteristics can in principle be obtained by making the line width of each of the line-shaped dielectric multilayer films less than the wavelength of the visible light, preferably 0.1 μm or less. Further, for its manufacturing method, a patterning technology for a super LSI can be applied to manufacture the periodic recesses and protrusions using dielectric multilayer films. Examples of known patterning technology include wet etching using an etchant such as hydrofluoric acid, dry etching such as reactive ion etching, or the like.
However, when a polarizing beam splitter is manufactured using the line-shaped dielectric multilayer films shown in FIG. 6, a wet process will be performed in an etching step or washing step, as described above. Further, a drying step is required after this wet process. Thus, a processing liquid flowing into the recesses during the above steps pulls the protrusions by a capillary force, so that a stress is applied to the dielectric multiplayer films as the protrusions to incline them. This tendency is significant when in the recesses and protrusions formed by the line-shaped dielectric multilayer films, the thickness of the dielectric multilayer film is relatively greater as compared with the width of the recess, i.e., the aspect ratio of the recesses and protrusions is high. This tendency becomes very significant particularly when the aspect ratio is 4 or more.
As a manufacturing method for avoiding this, Japanese Patent Application Laid-Open No. 2001-165568 discloses a drying method utilizing a supercritical state. According to this method, a processed pattern is immersed into normal hexane as a nonpolar solvent, and then sealed together with liquid carbon dioxide in a reaction chamber, so that the normal hexane is replaced with the liquid carbon dioxide to dry the processed pattern. This method is effective in preventing defects in the pattern. However, the inner pressure of the reaction chamber must be set to 7.5 MPa, thus requiring a high pressure container. Further, an organic solvent is required as a nonpolar solvent, which may pollute the environment.