JP4051747B2 - Manufacturing method of diffraction element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to the production how diffraction element.
[0002]
[Prior art]
Has been conventionally a diffraction element using a liquid crystal polymer was made work as follows. That is, an alignment treatment was performed on a transparent substrate surface such as glass, and a liquid crystal was thinly applied on the transparent substrate surface and cured by photopolymerization to obtain a polymer liquid crystal thin film. This polymer liquid crystal thin film was formed by subjecting a lattice having an uneven cross section to dry etching or the like, and filling the uneven portion with an isotropic medium (referred to as an optical isotropic medium). This diffractive element is a polarizing diffractive element having different diffraction efficiency depending on the polarization direction of incident light because the polymer liquid crystal thin film has birefringence.
[0003]
If the refractive index (n _s ) of the isotropic medium of the diffractive element is equal to the ordinary refractive index (n _o ) or extraordinary refractive index (n _e ) of the polymer liquid crystal thin film that is a birefringent medium, equal refraction The diffractive element does not function for polarized light in the rate direction.
For example, when the n _s = n _o, the diffraction element is transmitted showed high transmittance does not diffract the polarization of the ordinary refractive index direction. On the other hand, the diffractive element functions for polarized light in the direction of extraordinary refractive index and high diffraction efficiency is obtained.
[0004]
When this polarization type diffraction element is used in an optical head device, a direction of linearly polarized light when passing through the diffraction element by inserting a quarter-wave plate between the diffraction element and an optical disk as an optical recording medium The reciprocation efficiency can be increased by rotating 90 degrees on the forward and return paths.
[0005]
However, this time the polymer liquid crystal of the thin film crystal was applied as a monomer state before polymerization substrate subjected to orientation treatment in the work made, it is difficult to stabilize the alignment state obtained. If the orientation state is not stable, the substantial birefringence state after polymerization is changed, the refractive index is not stable, the desired diffraction efficiency cannot be obtained, and the diffraction element cannot be obtained with a high yield. In addition, it is difficult to control the film thickness of the polymer liquid crystal. When the film thickness varies, the diffraction efficiency varies, leading to a decrease in the manufacturing yield of the diffraction element.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-described problem of the prior art, it is to provide a manufacturing how diffraction element using a polymer liquid crystal film to the new.
[0007]
[Means for Solving the Problems]
The present invention relates to a method for manufacturing a diffraction element in which a grating having a concavo-convex section is formed on a polymer liquid crystal thin film and an isotropic medium is filled in the concavo-convex grating portion, and at least one of two opposing substrates is opposed. The surface is subjected to alignment treatment, a spacer and a liquid crystal to be a polymer liquid crystal thin film are sandwiched between the opposing substrates, the liquid crystal is aligned and cured to form a polymer liquid crystal thin film, and then at least one substrate is removed. A diffraction element manufacturing method is provided, wherein a grating having a concavo-convex section is formed in the polymer liquid crystal thin film, and an isotropic medium is filled in the concavo-convex grating portion.
[0008]
Further, the present invention provides a method for manufacturing the above-described diffraction element, wherein after the alignment treatment is performed on the two opposing substrates, a mold release treatment is performed on the facing surface of at least one of the substrates .
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, when the alignment treatment is performed on the opposing surfaces of the two opposing substrates, that is, the surface in contact with the liquid crystal, it may be applied to one substrate or two substrates. It is preferable to apply it to the substrate because it is easy to control the molecular orientation of the liquid crystal. The alignment treatment used here is typically a rubbing treatment on a substrate surface or a polyimide film attached to the surface, a polyamide film polymer film or an inorganic film such as SiO ₂ , or an oblique deposition process of SiO. is there.
[0010]
The liquid crystal for forming the polymer liquid crystal thin film used in the present invention is a composition such as a monomer, oligomer or other reactive compound exhibiting liquid crystallinity, and the liquid crystal is aligned as a composition by alignment treatment on the substrate. Can be used. In the following description, the liquid crystal monomer will be described as representative.
[0011]
A spacer is arranged between the two substrates facing each other, the substrates are opposed to each other with a predetermined interval, a liquid crystal monomer is injected into the gap, and the liquid crystal monomer is polymerized and cured by means of curing. A molecular liquid crystal thin film is used.
[0012]
As a means for curing the monomer of the liquid crystal, there are methods such as irradiation with light such as visible light and UV (ultraviolet) light, and heating methods. In particular, the curing method for irradiation with light such as visible light and UV light is a substrate. This is preferable because it can be directly performed. Accordingly, here, it is assumed that the monomer of the liquid crystal is polymerized and cured by light irradiation. In order to clarify that the liquid crystal is not polymerized and not polymerized, the term “liquid crystal monomer” is used instead of “liquid crystal”.
[0013]
An example of the manufacturing process of the polymer liquid crystal thin film is shown in FIG. FIG. 1A is a side view showing two substrates facing each other with a predetermined interval, and FIG. 1B is a diagram in which a liquid crystal monomer is injected between the two substrates facing each other, UV light or the like. It is a side view which shows a mode that the liquid crystal monomer was made into the polymer liquid crystal thin film by irradiating the light of FIG. 1, and FIG.1 (c) is a side view which shows a mode that the board | substrate of one of two opposing substrates was removed. is there. In the figure, 1 is a polymer liquid crystal thin film, 2 is a substrate, 11 is an alignment treatment film, and 12 is a spacer.
[0014]
As a liquid crystal monomer injection method, a vacuum injection method may be employed, or a method utilizing a capillary phenomenon may be performed at atmospheric pressure. In this case, prior to this, spacers may be spread on the first substrate in advance and then the second substrate may be laminated. Alternatively, after dropping a mixture of a spacer and a liquid crystal monomer onto the first substrate, the second substrate may be stacked and pressed.
[0015]
Further, after dispersing spacers on the first substrate, a liquid crystal monomer may be dropped and the second substrate may be stacked and pressed to form a thin film. As the substrate, a transparent glass plate, a plastic plate, or the like can be used, but a glass plate is preferable in terms of excellent hardness and durability.
[0016]
Then, a means for keeping the distance constant between the opposing substrates and a liquid crystal monomer to be a polymer liquid crystal thin film are sandwiched. The liquid crystal monomer to be used is preferably selected from esters such as acrylic acid or methacrylic acid.
Among these, an acrylic acid type material which is a material (side chain polymer liquid crystal) which is polymerized by adding a reactive group to a liquid crystal monomer and polymerizing is preferable. In addition to the high birefringence of the material itself, this polymer liquid crystal has an excellent feature that the monomer of this liquid crystal reacts sensitively to the alignment treatment of the substrate and tends to increase the birefringence after polymerization. Have.
[0017]
If there is a distribution in the thickness of the polymer liquid crystal thin film, the diffraction efficiency varies as described above, which is not preferable. Accordingly, a spacer is used as a means for keeping the distance between the opposing substrates constant. This spacer can function as long as it is made of glass, alumina, silica, or other inorganic or plastic spherical, granular, or fibrous spacers that have rigidity and durability. However, glass spacers are particularly rigid and durable. It is excellent and preferable. Usually, the thickness of the polymer liquid crystal thin film is 1 to 5 μm.
[0018]
As this glass, spherical SiO ₂ glass is preferable, and when the diameter is 5 μm or less, it is preferable that the number is 10 to 200,000 per 1 cm ² . If the number is less than 10, it is difficult to keep the distance between the substrates constant. If the number is more than 200,000, light scattering is caused, and stray light may not be generated when the optical head device is mounted. The number is preferably from 1,000 to 10,000, and in this case, the above-mentioned problems can be effectively avoided.
[0019]
Visible light, UV light or the like is used as described above for light irradiation performed after aligning the liquid crystal monomer, but UV light is preferred for efficient curing. By performing light irradiation in this manner, the monomer of the liquid crystal can be cured while maintaining the alignment state.
[0020]
After curing, at least one of the two opposing substrates is removed (FIG. 1C). In this case, in order to prevent the polymer liquid crystal thin film from adhering to the substrate to be removed, it is preferable to perform mold release treatment on the inner surface of the substrate to be removed, that is, the surface in contact with the liquid crystal monomer.
When the alignment treatment is applied to the inner surface of one of the two substrates facing each other, the release effect is applied to the inner surface of the substrate not subjected to the alignment treatment. It is preferable to increase the value.
As the release agent used for the release treatment, a fluorosilane-based, fluorine-containing polymer having a fluorine-containing aliphatic ring structure, or the like can be used.
[0021]
A diffractive element is obtained by forming a grating having a concave-convex cross section in the polymer liquid crystal thin film thus manufactured and filling the concave-convex portion with an isotropic medium.
That is, a lattice having an uneven cross section is formed on the polymer liquid crystal thin film by an etching method using photolithography or a pressing method using a mold having a lattice shape. Uneven grating portion the cross corrugated grating is formed, the refractive index is filled with equal isotropic medium to the ordinary refractive index of the polymer liquid crystal film (n _o) or the extraordinary refractive index (n _e) Harden. As the isotropic medium, for example, a photopolymerizable acrylic resin or epoxy resin can be used.
[0022]
By selecting the refractive index in this way, the diffractive element may or may not function as a diffraction grating depending on the polarization direction of light passing through the diffractive element.
That is, the incident if the refractive index is filled into the concave-convex portion equal isotropic medium to the ordinary refractive index of the polymer liquid crystal film (n _o), the light linearly polarized in a direction giving the ordinary refractive index of the diffraction element in the diffraction element In this case, there is no difference in refractive index between the polymer liquid crystal thin film and the isotropic medium, so that a diffraction effect does not occur.
[0023]
However, when light linearly polarized in a direction giving an extraordinary refractive index that forms an angle of 90 degrees with this direction is incident on the diffraction element, there is a refractive index difference between the polymer liquid crystal thin film and the isotropic medium. Produces an effect. The same applies when the concavo-convex portion is filled with an isotropic medium equal to the extraordinary light refractive index (n _e ).
[0024]
There is a method of filling the isotropic medium polymer ordinary refractive index of the liquid crystal film (n _o) also extraordinary refractive index (n _e) in the not equal refractive index to the uneven portion. In this case, when the optical disk that is an optical recording medium has birefringence, the intensity of the signal light reflected by the optical disk is not lowered by combining with the diffraction element shown in FIG.
[0025]
The diffractive element manufactured by the manufacturing method of the present invention has good alignment in one direction of the molecular axis of the polymer liquid crystal, so the refractive index is uniform and high in the diffractive element, and the thickness of the polymer liquid crystal thin film is uniform. Therefore, it has an excellent characteristic that high and uniform diffraction efficiency can be obtained.
[0026]
Further, as shown in FIG. 2 which is a side view showing an example of the diffraction element according to the present invention, the diffractive element can be formed without completely removing the polymer liquid crystal thin film 1 in the recessed portion while remaining on the substrate 2. In this case, when the light passes through the remaining polymer liquid crystal thin film, the polarization direction may change depending on the incident angle or the like. Therefore, as shown in FIG. 4 which is a side view showing another example of the diffractive element according to the present invention, the polymer liquid crystal thin film 1 in the concave portion is completely removed, that is, the concave portion penetrates the polymer liquid crystal thin film 1 and the substrate 2. It is preferable to expose the surface. 2 and 4, 3 is an isotropic medium.
[0027]
In FIG. 5, which is a side view showing another example of the diffraction element according to the present invention, a quarter-wave plate 10 is provided integrally with the diffraction element. 1, 2 and 3 are the same as described above.
[0028]
Furthermore, the diffraction element manufactured by the manufacturing method of the present invention is mounted on the optical head device. That is, as a diffractive element of an optical head device according to the present invention, light emitted from a semiconductor laser is guided to an optical recording medium, reflected light from the optical recording medium is diffracted by a diffractive element, and the diffracted light is detected by a photodetector. The diffraction element manufactured by the manufacturing method is used.
[0029]
FIG. 3 is a side view showing an outline when the diffraction element of the present invention is used as a hologram beam splitter in an optical head device. The light emitted from the semiconductor laser 5 as the light source passes through the diffraction element 4 as the hologram beam splitter, is condensed on the optical disc 8 by the objective lens 7, and the reflected light from the optical disc 8 passes through the objective lens 7 again. It is diffracted by the hologram beam splitter and reaches the light receiving element 6.
[0030]
By inserting the quarter wavelength plate 10 between the hologram beam splitter and the optical disk 8, the polarization direction of the linearly polarized light emitted from the semiconductor laser 5 can be rotated by 90 degrees in the forward path and the backward path, The transmittance is high for the light in the forward polarization direction, and the diffraction efficiency is high for the light in the polarization direction of the return path, so that the light utilization efficiency can be increased.
[0031]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
Side views of each process of the method for producing a polymer liquid crystal thin film are shown in FIGS. 1 (a), (b) and (c).
[0032]
First, as shown in FIG. 1 (a), a polyimide thin film is formed on the inner surfaces of two glass substrates 2 having a diameter of 3 inches facing each other, and is then rubbed in one direction parallel to the two substrates. By rubbing alignment treatment, an alignment treatment film 11 was obtained. A fluorosilane release agent was applied (not shown) on the alignment treatment film 11 of one of the two glass substrates 2 facing each other.
[0033]
On the other substrate, spherical spacers 12 made of SiO ₂ having a diameter of 2.0 μm were sprayed at a density of 6000 pieces / cm ² , and the two substrates were bonded so that the distance between the inner surfaces was 2 μm. Next, as shown in FIG. 1B, acrylic acid-based liquid crystal monomer was injected into the gap between the two substrates by vacuum injection.
[0034]
When the alignment state of the liquid crystal was observed in this state, it was observed that the molecular axes of the liquid crystal monomers were aligned in the rubbing direction, and it was confirmed that the alignment state was good. Next, the polymer liquid crystal thin film 1 was formed by irradiating 600 mJ of UV light through a glass substrate to photopolymerize and cure the liquid crystal monomer. The alignment state of the polymer liquid crystal was also uniform and good.
[0035]
Furthermore, as shown in FIG.1 (c), the board | substrate of which the mold release process was performed was removed. As a result, the polymer liquid crystal on the substrate that had not been subjected to the mold release treatment remained on the substrate without peeling off, and the polymer liquid crystal thin film 1 could be formed on this substrate side. The orientation state was as good as before the substrate was removed. The film thickness of the polymer liquid crystal thin film 1 was about 2 μm, and a uniform film thickness as designed could be obtained.
[0036]
Thereafter, the polymer liquid crystal thin film 1 which is a birefringent material on the substrate is formed into a lattice having an uneven cross section (FIG. 4) by a photolithography etching method and having a longitudinal direction parallel to the rubbing direction. The isotropic medium 3 was filled and cured.
[0037]
The refractive index of the isotropic medium 3 at this time was equal to the ordinary refractive index of the polymer liquid crystal thin film 1. Then, the substrate was cut into an outer dimension of 4 mm × 4 mm to obtain a hologram beam splitter which is a polarizing diffraction element.
[0038]
The hologram beam splitter thus manufactured was incorporated as the diffraction element 4 of the optical head device shown in FIG. The hologram beam splitter mounted on this optical head device shows a high transmittance of 95% with respect to the linearly polarized light emitted from the semiconductor laser 5 which is the outgoing light, and is reflected by the optical disk 8 which is the backward light. A high ± first-order diffraction efficiency of 36% was exhibited for the light transmitted through the / 4 wavelength plate 10 and polarized light whose polarization direction was rotated by 90 degrees.
[0039]
Therefore, 0.95 × 0.36 = 0.34, that is, a high light utilization efficiency of 34% can be obtained, and the diffraction efficiency of this hologram beam splitter is uniform in the plane.
[0040]
【The invention's effect】
By adopting the production method of the present invention, the liquid crystal monomer is well and stably oriented, and further, the film thickness of the prepared polymer liquid crystal thin film has a refractive index. It is stable and high, and the film thickness can be made uniform.
Therefore, the diffractive element manufactured by the manufacturing method of the present invention in which an isotropic medium is filled in an uneven cross-sectional grating manufactured using this polymer liquid crystal thin film has a high diffraction efficiency and a uniform diffraction efficiency. It has the characteristic which is.
[0041]
In addition, according to the present invention, a polymer liquid crystal thin film having a uniform film thickness can be easily obtained, so that the yield of manufacturing diffraction elements can be improved.
Furthermore, by incorporating the diffraction element manufactured by the manufacturing method of the present invention into the optical head device, the optical head device becomes a device with high light utilization efficiency and stable diffraction efficiency.
[Brief description of the drawings]
FIG. 1 is a side view showing an example of a process for producing a polymer liquid crystal thin film according to the present invention. (A) The side view which shows the board | substrate which provides predetermined spacing and opposes. (B) The side view which shows a mode that the monomer of the liquid crystal was inject | poured between the board | substrates which opposes, and it made the polymer liquid crystal thin film. (C) The side view which shows a mode that one board | substrate was removed from the board | substrate which opposes.
FIG. 2 is a side view showing an example of a diffraction element according to the present invention.
FIG. 3 is a side view schematically showing the head device of the present invention.
FIG. 4 is a side view showing another example of the diffraction element according to the present invention.
FIG. 5 is a side view showing another example of the diffraction element according to the present invention.
[Explanation of symbols]
1: Polymer liquid crystal thin film 2: Substrate 3: Isotropic medium 4: Diffraction element (hologram beam splitter)
5: Semiconductor laser 6: Light receiving element 7: Objective lens 8: Optical disk 10: 1/4 wavelength plate 11: Orientation treatment film 12: Spacer

Claims (2)

In a method for manufacturing a diffractive element in which a lattice having a concavo-convex shape is formed on a polymer liquid crystal thin film and an isotropic medium is filled in the concavo-convex lattice portion, at least one opposing surface of two opposing substrates is subjected to an alignment treatment And sandwiching a spacer and a liquid crystal to be a polymer liquid crystal thin film between the opposing substrates, aligning and curing the liquid crystal to form a polymer liquid crystal thin film, and then removing at least one substrate A method for manufacturing a diffraction element, comprising: forming a grating having a concave-convex section in a liquid crystal thin film, and filling the concave-convex grating portion with an isotropic medium.   2. The method of manufacturing a diffraction element according to claim 1, wherein after the alignment treatment is performed on the two opposing substrates, a mold release process is performed on the facing surface of at least one of the substrates.

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