JP4363675B2 - Manufacturing method of liquid crystal diffraction element and optical head device using this element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device for reading and recording information on an optical recording medium such as an optical disk such as a CD or CD-ROM, or a magneto-optical disk.
[0002]
[Prior art]
Conventionally, an isotropic grating is used as a diffraction element used in an optical head device. In this isotropic grating, a rectangular grating (relief type) is formed on glass or plastic by dry etching, injection molding, or the like, and light is diffracted by this grating to have a function as a beam splitter.
However, when this isotropic lattice is used in an optical head device, the light utilization efficiency is low. Specifically, the forward route is at most 50%, the return route is at most 20%, and the limit of about 10% is the round trip.
[0003]
In order to improve this efficiency, a liquid crystal diffraction element having an anisotropic grating is used. As this liquid crystal diffractive element, a lattice-like unevenness is provided on one inner surface of an opposing substrate and liquid crystal is sealed between both substrates, or a lattice-like planar periodic transparent electrode is provided on the substrate. It is known to form a diffraction grating by applying a voltage to a transparent electrode to change the alignment direction of the liquid crystal between a portion with the transparent electrode and a portion without the transparent electrode.
[0004]
In addition, the present inventors have developed a liquid crystal diffractive element using a photopolymerizable liquid crystal having both a structure exhibiting liquid crystallinity and a photopolymerizable structure. It has been proposed to irradiate the liquid crystal with ultraviolet light or the like while sandwiching unpolymerized liquid crystal between the substrates and applying a voltage to the planar periodic transparent electrodes provided on both substrates ( Japanese Patent Application No. 8-100369).
[0005]
FIG. 3 shows a method for manufacturing the proposed liquid crystal diffraction element. That is, the alignment films 34A and 34B for horizontal alignment are formed of polyimide or the like on the surface where the opposing substrates 31A and 31B are in contact with the liquid crystal 33, and the planar periodic transparent electrodes 32A and 32B provided on the opposing substrate are formed. The installation position is a position where the transparent electrodes 32A and 32B face each other. By applying a voltage to the transparent electrodes 32A and 32B by the voltage generator 35, the liquid crystal 33 is aligned perpendicular to the surfaces of the substrates 31A and 31B, and in a region where the planar transparent electrodes are not installed, the liquid crystal 33 is Therefore, the entire liquid crystal 33 is photopolymerized by ultraviolet irradiation light 37 to create an alignment state of the cured liquid crystal whose orientation direction periodically changes in a plane, thereby producing a liquid crystal diffraction element. It was.
[0006]
On the other hand, the conventional optical head device has a light source, a collimating lens for collimating the diverging light from the light source, a condensing means for forming collimated light collimated by this lens into a minute spot, and an optical disc in the state of minute spot light. After the reflected light containing the signal of (1) is returned to the condensing part, the light beam separating means for changing the traveling direction thereof, and after being deflected, separated and condensed by the light beam separating means, the separated light is received as signal light. Consists of components such as a light receiving element. Normally, the light source, the light beam separating means, and the light receiving element are installed at the closest positions.
[0007]
[Problems to be solved by the invention]
An anisotropic lattice for forming irregularities on the inner surface of the substrate has a problem of low productivity because the irregularity forming process and the liquid crystal injection process are necessary.
The liquid crystal diffractive element that forms a transparent electrode that is periodic in plane has a large volume due to the addition of a voltage generator, and the configuration becomes complicated. In addition, there was a problem that the electric field leaked and the boundary of the diffraction grating was blurred.
[0008]
Also, in the case of the proposed method, since the planar transparent electrode is used, the grating of the prepared liquid crystal diffraction element has a cause such as electric field leakage to the liquid crystal at the periphery of the transparent electrode. The pitch was not fine enough.
[0009]
[Means for Solving the Problems]
The present invention was made to solve the above-mentioned problems, and in the method for producing a liquid crystal diffraction element using a photopolymerizable liquid crystal, a photopolymerization initiator and a photopolymerization inhibitor are added to the liquid crystal, An alignment film on the surface of at least one of the opposing transparent substrates that is in contact with the liquid crystal, sandwiching the liquid crystal so that the curing speed is reduced with respect to weak light irradiation compared to a liquid crystal that does not contain a photopolymerization inhibitor A transparent electrode is provided on the entire surface of one side of both substrates, and a photomask having a periodic light-shielding pattern is disposed on the substrate surface while applying a voltage between the transparent electrodes of both substrates or Without applying a voltage, the liquid crystal is irradiated with parallel light through a photomask to cure only the liquid crystal in the area that is exposed to the planar periodic light, and then change the voltage application state between the transparent electrodes. By irradiating the liquid crystal with light To cure the liquid crystal of the remaining portion which is not cured, the alignment direction of the liquid crystal cured to provide a method of manufacturing a liquid crystal diffraction element and forming a dimensionally periodically varying the diffraction grating.
[0010]
Et al is to provide an optical head device using a liquid crystal diffraction element created by any of the manufacturing methods described above.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the following description, “planarly periodic” is simply expressed as “periodic”. Further, “parallel” or “perpendicular” means “parallel to the substrate surface” or “perpendicular to the substrate surface” unless otherwise specified.
[0012]
In the present invention, the transparent electrode formed on the opposing substrate is a solid electrode provided on only one side of the substrate. In addition, there are roughly two methods for manufacturing a liquid crystal diffraction element. In each method, a process for forming a periodic alignment of liquid crystals performed by light irradiation while applying a voltage between the electrodes is divided into two steps. Divide. The liquid crystal used here has a photopolymerization initiator and a photopolymerization inhibitor added to the liquid crystal so that the curing speed is reduced with respect to weak light irradiation compared to a liquid crystal not containing a photopolymerization inhibitor. It is a liquid crystal.
[0013]
As a first manufacturing method, in the first step, a portion of the liquid crystal irradiated with light through a photomask having a periodic light-shielding pattern while applying no voltage between the electrodes and aligning the entire liquid crystal in parallel Is cured in the horizontal direction, and in the next second step, the cured portion is aligned while the non-cured portion of the liquid crystal is vertically aligned while applying a vertical voltage to the entire liquid crystal including the cured portion. The entire liquid crystal is irradiated with light and further cured.
[0014]
As a second manufacturing method, in the first step, the liquid crystal is exposed to light through a photomask having a periodic light-shielding pattern while applying a voltage perpendicular to the substrate surface and aligning the entire liquid crystal vertically. The part is first cured in the vertical direction, and in the next second step, light is applied to the entire liquid crystal including the cured part while horizontally applying the uncured liquid crystal part without applying a voltage between the electrodes. To cure further.
[0015]
Any of the above manufacturing methods has the same effect, and both of them can provide a liquid crystal diffraction element in which the orientation direction of the cured liquid crystal changes periodically.
As the photomask used here, a photomask having a periodic unidirectional lattice-shaped light shielding pattern may be used, and ultraviolet light, visible light, or the like may be used as irradiation light for curing the liquid crystal.
[0016]
The alignment film is preferably subjected to an alignment treatment in the horizontal direction. As the alignment treatment method in the horizontal direction with respect to the substrate, a rubbing method, an oblique vapor deposition method such as silica, or the like may be used.
The alignment treatment by rubbing in the horizontal direction with respect to the substrate may be performed perpendicularly or parallel to the grating direction within the photomask plane, but considering the high transmittance, it is preferable to perform the alignment process perpendicularly to the grating direction. preferable. In any case, when a periodic structure in the alignment direction of the liquid crystal is created, it functions as a diffraction grating when polarized light parallel to the rubbing direction is incident, and functions as a diffraction plate when it does not function as a diffraction grating when incident perpendicularly polarized light.
[0017]
Further, in order to reduce multiple reflection due to incident light on the surface of the substrate, it is preferable to form an antireflection film on the outer surface of the substrate, and it is more preferable to form it on the inner surface of the substrate facing the liquid crystal.
[0018]
Further, since the transparent electrode for voltage application is a solid electrode provided on the entire surface of one side of the substrate, there is no problem of electric field leakage to the liquid crystal region between the electrodes because there is no break. Therefore, the liquid crystal aligned in the horizontal direction is not dragged in the vertical direction by the leakage electric field. The electrode is a transparent conductive film, and a metal thin film, an ITO film, or the like may be used.
Here, a transparent substrate such as glass or plastic can be used as the substrate constituting the liquid crystal diffraction element.
[0019]
The periodic light shielding pattern of the photomask forms one unit with the light shielding portion and the opening, and the length of this unit, that is, the pitch of the light shielding pattern is the same everywhere. Further, by using a light shielding pattern in which the ratio of the opening width and the pitch of the pattern is 20% or more and 40% or less, it is possible to avoid overlapping of light bundles that have passed through the transparent portion that is the opening of the light shielding pattern, Moreover, it is preferable because the intensity of unnecessary high-order diffracted light can be reduced.
In addition, it is preferable to dispose an optical system for further collimating the light beam that has been separated from the photomask and the substrate and passed through the opening of the photomask. Here, the pitch of the light shielding pattern of the photomask may be appropriately selected and used within a range of 5 to 50 μm.
[0020]
The liquid crystal to be used is preferably selected from esters such as acrylic acid or methacrylic acid. It is preferable that one or more, particularly 2 or 3 phenyl groups are contained in the alcohol residue forming the ester. Furthermore, one cyclohexyl group may be contained in the alcohol residue forming the ester. In order to widen the temperature at which the liquid crystal can exist, two or more components can be mixed and used.
[0021]
As a reference, the region of the liquid crystal to be cured can be limited by using a photopolymerization initiator that becomes transparent by light irradiation when light is irradiated in the first step. That is, by using this photopolymerization initiator, when light is incident from the substrate surface through the photomask, the transparency gradually proceeds from the region of the liquid crystal where light is first incident, and transparent while preventing light diffusion. Light is guided to the converted area, and light irradiation can be performed while limiting the liquid crystal area.
[0022]
Examples of photopolymerization initiators that are considered to be transparent by light irradiation include benzoin isopropyl ether, camphorquinone, 2-methyl-1- [4- (methylthio) phenyl] -2-morpholinopropan-1-one And o-chlorohexaaminobiimidazole. This photopolymerization initiator is preferably added in an amount of 0.1 to 3 wt% in the liquid crystal.
[0023]
In the present invention, in addition to the photopolymerization initiator, a photopolymerization inhibitor is mixed into the liquid crystal so as to reduce the speed of curing of the liquid crystal with weak light. For weak leaking light in the boundary region between the part of the liquid crystal that was exposed to light and the part that did not hit the light through the photopolymerization, the function of the photopolymerization initiator was reduced by prohibiting photopolymerization. Curing at a specific part can be caused. That is, the photopolymerization initiator acts to cause the photopolymerization in the portion irradiated with the strong light, and the photopolymerization inhibitor acts in the weak light portion to prevent the photopolymerization. As the photopolymerization inhibitor, 2,4,6-tri-tert-butylphenol, hydroquinone, or the like can be used. This photopolymerization inhibitor is preferably added in an amount of 0.5 to 3 wt% in the liquid crystal.
[0024]
In the case of performing the light irradiation of the present invention, it can be performed through a photomask using a reduction optical system that reduces the irradiation light, but since the depth of focus becomes shallow, a photomask using parallel light that does not reduce the irradiation light is used. Is preferred. In addition, a method using a bright and dark grid pattern in which the pitch of interference fringes generated by interference of two laser beams is freely used without using a photomask can also be applied. Is preferred.
[0025]
By using the liquid crystal diffraction element according to the present invention as the light beam separation element of the optical head device, an optical head device with high light utilization efficiency can be obtained. The liquid crystal diffractive element may be installed at any location as long as the return light from the optical disk is deflected and can enter the detector.
[0026]
Further, the liquid crystal diffraction element according to the present invention can be incorporated in a hologram laser unit having at least a semiconductor laser as a light source and a photodiode as a detector. Accordingly, a hologram laser unit with high light utilization efficiency can be provided, and a light source, a detector, a light beam separation element, and the like can be integrated, and the optical head device can be made compact.
[0027]
【Example】
"Example 1" (Reference example)
A method for producing the liquid crystal diffraction element of Example 1 is shown in FIG. a) represents light irradiation in the first step, and b) represents light irradiation in the second step.
A transparent conductive film using an ITO film having a thickness of 50 nm was formed by sputtering on a substrate 11A using glass having a thickness of 0.5 mm. A solid transparent electrode 12A using an ITO film is provided on the substrate 11A, and a solid transparent electrode 12B using an ITO film is formed on the opposing substrate 11B, and the opposing surfaces of both substrates (the transparent electrode of the ITO film) The polyimide alignment films 14A and 14B having a thickness of 60 nm were formed on the surface of the substrate, and rubbed with a rubbing cloth in a direction horizontal to the substrate surface and perpendicular to the lattice direction of the photomask 16.
[0028]
Between the two substrates 11A and 11B, the rubbing direction is opposite to 180 °, both substrates are opposed to each other, a sealing material mixed with a spacer having a particle diameter of 3 μm is arranged around the periphery, and thermocompression bonding is performed. Produced a cell structure of approximately 3 μm. Between the two substrates 11A and 11B, 4 ′-{ω-acryloyloxy) alkyloxy} cyanobiphenyl and p- [4- {ω- (acryloyloxy) alkyloxy}] benzoic acid p′-n— Liquid liquid crystal 13 mainly composed of alkyloxyphenyl ester was injected. Furthermore, 1 wt% of benzoin isopropyl ether as a photopolymerization initiator was added to the liquid crystal 13 to obtain a curable liquid crystal composition by ultraviolet rays.
[0029]
In the light irradiation of the first step, the voltage generator 15 is used to apply a voltage of 10 V _rms and 100 Hz between the transparent electrodes 12A and 12B, and the liquid crystal is aligned vertically, while the substrate is interposed through the photomask 16. The irradiation light 17 which is an ultraviolet ray of about 80 mJ was applied toward 11A. Here, the light-shielding pattern of the photomask 16 used for light irradiation was in a lattice shape, the opening width was 3 μm, and the periodic pitch was 8 μm.
A projection aligner (not shown) is arranged between the photomask 16 and the substrate 11A to further collimate the light beam 18 that has passed through the photomask 16.
[0030]
The liquid crystal in the irradiated area was hardened by photopolymerization, and when the application of voltage was stopped, the change reaction in the alignment direction was lower than that in the non-irradiated area, and no return to the horizontal direction was observed.
In a state where the application of voltage is stopped, photopolymerization is performed by irradiating irradiation light 17, which is ultraviolet light of 1500 mJ, on the entire surface of the liquid crystal from the side of the substrate 12A sandwiching the horizontally aligned liquid crystal through the photomask as irradiation in the second step. Thus, the liquid crystal was cured to fix the structure in which the alignment direction of the liquid crystal changes periodically. Then, the outer peripheral part of the board | substrate was cut | disconnected and the liquid crystal diffraction element of an outer dimension 5.0 mm square was produced.
[0031]
The produced liquid crystal diffractive element was a laser beam having a wavelength of 650 nm and exhibited a transmittance of 90% or more for linearly polarized light perpendicular to the rubbing direction of the substrate. On the other hand, incident light was diffracted with respect to linearly polarized light parallel to the rubbing direction, and a diffraction efficiency of primary light of about 35% in total was obtained.
[0032]
"Example 2"
A method for producing the liquid crystal diffraction element of Example 2 is shown in FIG. a) represents light irradiation in the first step, and b) represents light irradiation in the second step.
A solid transparent electrode 22A using an ITO film having a thickness of 50 nm was formed on a substrate using glass having a thickness of 0.5 mm by sputtering. Reflection of 1% or less with respect to 650 nm which is a wavelength when using a liquid crystal diffraction element and 365 nm which is a wavelength of light irradiation by vapor-depositing an optical multilayer film on the surface opposite to the ITO film forming surface of the substrate 21A An antireflection film 29 having a refractive index was formed. After forming 60 nm-thick polyimide alignment films 24A and 24B on the opposing ITO film-forming surfaces of the two substrates 21A and 21B, horizontal rubbing with a rubbing cloth was performed in the same direction as in Example 1. .
[0033]
The two substrates 21A and 21B have the rubbing direction opposite to 180 °, both substrates 21A and 21B face each other, and a sealing material mixed with a spacer having a particle size of 3 μm is placed around the periphery and thermocompression bonded to provide a gap of about 3 μm. The cell structure was fabricated. Between these two substrates 21A and 21B, 4 ′-{ω-acryloyloxy) alkyloxy} cyanobiphenyl and p- [4- {ω- (acryloyloxy) alkyloxy}] benzoic acid p′-n— Liquid liquid crystal 23 mainly composed of alkyloxyphenyl ester was injected.
[0034]
Further, 1 wt% benzoin isopropyl ether is added to the liquid crystal 23 as a photopolymerization initiator, and 0.5 wt% 2,4,6-tri-tert-butylphenol is added as a photopolymerization inhibitor, and ultraviolet light is used. A curable liquid crystal composition was obtained. Irradiation light 27, which is ultraviolet light, is applied from the upper surface of the substrate 21A, and parallel ultraviolet light having a wavelength of 365 nm generated through a photomask 26 having a periodic light-shielding pattern in a lattice shape with a periodic pitch of 8 μm and an opening width of 3 μm. The light beam 27 was applied to the liquid crystal 23 for about 120 mJ.
Also in this case, as in Example 1, a projection aligner for further collimating the light beam 28 that passed through the photomask 26 was disposed between the photomask 26 and the substrate 21A (not shown).
[0035]
The liquid crystal in the irradiated part progressed by the ultraviolet irradiation in the first step, and the alignment reaction of the liquid crystal in the electric field direction due to voltage application was lower than that in the non-irradiated part. At this time, since the intensity of the multiple reflected light from the substrates 21A and 21B of the irradiated ultraviolet rays is reduced by the antireflection film 29, a noticeable blur of the boundary line between the irradiated part and the non-irradiated part based on the multiple reflection is seen. There wasn't.
[0036]
In addition, light wraparound occurs at the boundary between the irradiated part of the liquid crystal that is exposed to light through the photomask and the non-irradiated part that is not exposed, but the light intensity is weak, so the photopolymerization inhibitor added in this boundary region The photopolymerization of the liquid crystal does not proceed due to the action of this (this composition does not proceed with the weak light of 40 mJ or less), and the liquid crystal can be selectively cured except for the liquid crystal portion where the light intensity is weak. did it.
[0037]
In this state, a voltage generator 25 is used to apply a voltage of 15 V _rms and 100 Hz between the transparent electrodes 22A and 22B, and in the first step, the liquid crystal that was not irradiated with ultraviolet rays is vertically aligned and cured. The liquid crystal in the irradiated area where the light is advanced is kept aligned in parallel. With this voltage applied, the liquid crystal is cured by photopolymerization by applying 1500 mJ ultraviolet irradiation light 27 from the side of the substrate 22A holding the liquid crystal as the ultraviolet irradiation in the second step, and the alignment direction of the liquid crystal is Fixed the structure of the periodically cured liquid crystal.
[0038]
Then, the outer peripheral part of the board | substrate was cut | disconnected and the liquid crystal diffraction element of an outer dimension 5.0 mm square was produced. The produced liquid crystal diffractive element was a laser beam having a wavelength of 650 nm and exhibited a transmittance of 90% or more for linearly polarized light perpendicular to the rubbing direction. On the other hand, incident light was diffracted with respect to linearly polarized light parallel to the rubbing direction, and a diffraction efficiency of primary light of about 40% in total was obtained.
[0039]
"Example 3"
The liquid crystal diffractive element prepared in Example 2 was disposed between the light detection unit of the optical head device and the collimating lens, and the quarter wavelength plate was disposed between the collimating lens and the condenser lens.
[0040]
Since the directional relationship between the semiconductor laser and the liquid crystal diffraction element is set so that the direction of the linearly polarized light emitted from the semiconductor laser having a wavelength of 650 nm is perpendicular to the rubbing direction of the liquid crystal diffraction element, the light emitted from the semiconductor laser is emitted from the liquid crystal diffraction element. The transmission rate was 90% in the outward path passing through. This light passes through the quarter-wave plate, becomes circularly polarized light, is reflected by the optical disc, and becomes circularly polarized return light that rotates in the opposite direction. The return light that has passed through the quarter-wave plate again changes from circularly polarized light to linearly polarized light. Thus, the polarization direction was rotated by 90 °, and the diffraction efficiency of the first-order diffracted light when passing through the liquid crystal diffraction element with the rubbing direction of the liquid crystal diffraction element parallel to the polarization direction was 40%.
[0041]
Accordingly, the light emitted from the semiconductor laser is reflected as reflected light from the optical disk, which is an optical recording medium, and as a result, the total of the first-order diffracted light is observed as signal light having an intensity of about 35%. Results were obtained.
[0042]
【The invention's effect】
When the liquid crystal sandwiched between the transparent substrates facing each other is irradiated with light through a photomask having a periodic light-shielding pattern, the light irradiation process is divided into two steps, and light is applied by changing the voltage application state to the liquid crystal. By performing irradiation, the boundary line of the hardened liquid crystal regions with different orientation directions can be clearly defined and a fine unidirectional lattice-like periodic pitch can be obtained. Therefore, a liquid crystal diffraction element with high light utilization efficiency can be easily obtained. Is obtained.
[0043]
Further, by using this liquid crystal diffractive element in an optical head device, it is possible to increase the light use efficiency when the semiconductor laser beam reciprocates through the liquid crystal diffractive element, and a high S / N can be obtained even in a low output laser device. Further, the optical head device itself can be reduced in size and weight, and the power consumption of the optical head device can be reduced.
[Brief description of the drawings]
1 is a cross-sectional view showing a method for producing a liquid crystal diffraction element of Example 1. FIG. a) Irradiation of light in the first step, b) Irradiation in the second step.
2 is a cross-sectional view showing a method for producing the liquid crystal diffraction element of Example 2. FIG. a) Light irradiation in the first step, b) Light irradiation in the second step.
FIG. 3 is a cross-sectional view showing a conventional method for manufacturing a liquid crystal diffraction element.
[Explanation of symbols]
11A, 11B, 21A, 21B: Substrates 12A, 12B, 22A, 22B: Transparent electrodes 13, 23: Liquid crystals 14A, 14B, 24A, 24B: Alignment films 15, 25: Voltage generators 16, 26: Photomasks 17, 27 : Irradiation light 18, 28: Light flux 29: Antireflection film

Claims (2)

In a method for producing a liquid crystal diffraction element using a photopolymerizable liquid crystal, a photopolymerization initiator and a photopolymerization inhibitor are added to the liquid crystal, and compared with a liquid crystal not containing a photopolymerization inhibitor, the light is not easily irradiated. An alignment film is provided on the surface of at least one of the opposing transparent substrates facing the liquid crystal so that the curing speed is reduced, and a transparent electrode is provided on the entire surface of one side of both substrates. A photomask having a periodic light shielding pattern is arranged on the substrate surface, and the liquid crystal is irradiated with parallel light through the photomask while applying a voltage between the transparent electrodes of both substrates or without applying a voltage. To cure only the liquid crystal in the part that has been periodically exposed to light, and then change the voltage application state between the transparent electrodes and irradiate the liquid crystal with light, so that the remaining uncured part is cured. The liquid crystal was cured and cured Method of manufacturing a liquid crystal diffraction element and forming a diffraction grating alignment direction of the crystal is changed in a plane periodically. An optical head device using a liquid crystal diffraction element created in claim 1 Symbol mounting method of manufacturing.

Images