JPH1010307A - Production of optical diffraction gating and optical head device formed by using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily produce the optical diffraction gratings having high utilization efficiency of one beam of diffracted light by diagonally exposing a photoresist and asymmetrically forming the flanks in the short side direction of the gratings of the photoresist made to remain by developing this photoresist, then subjecting the photoresist to dry etching. SOLUTION: The photoresist 3 is formed on the surface of the substrate 1 itself or on the surface of the substrate 1 provided with transparent material films 2, 4. This photoresist 3 is diagonally exposed and developed, by which the flanks in the short side direction of the gratings of the remaining photoresist 3 are formed asymmetrical. The photoresist is then dry-etched, by which the flanks in the short side direction of the gratings of the projecting parts made to remain by etching are made asymmetrical. The right and left flanks of the projection in the short side direction of the the stripes of the gratings of the transparent materials 2, 4 have different tilt angles and turn symmetrical with a center line. The optical diffraction gratings having such shapes vary in +1st order and -1st order in the light quantity of the diffracted light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for writing optical information on an optical recording medium such as a CD (compact disk), CD-ROM, video disk and the like and a magneto-optical disk and reading optical information. The present invention relates to a method for manufacturing an optical diffraction grating suitable for an optical head device and an optical head device using the same.

[0002]

2. Description of the Related Art Conventionally, an optical head device for writing optical information on an optical recording medium such as an optical disk and a magneto-optical disk and reading the optical information detects a signal light reflected from a recording surface of the optical recording medium. As an optical component for guiding (beam splitting) light to a portion, a device using a prism type beam splitter and a device using a diffraction grating or a hologram element have been known.

A diffraction grating or hologram element for an optical head device is formed by forming a rectangular grating (relief type) having a rectangular cross section on a glass or plastic substrate by a dry etching method or an injection molding method. To provide a beam splitting function.

In order to increase the light use efficiency over an isotropic diffraction grating having a light use efficiency of about 10%, it is conceivable to use polarized light. In order to use polarized light, a prism type beam splitter is combined with a λ / 4 plate to increase the efficiency of the forward path (the direction from the light source to the optical recording medium) and the return path (the direction from the optical recording medium to the photodetector). There was a way to increase the reciprocating efficiency.

However, the prism type polarizing beam splitter is expensive, and other methods have been sought. As one method, a method of using a flat plate of birefringent crystal such as LiNbO ₃ and forming an anisotropic diffraction grating on the surface to have polarization selectivity is known. However, the birefringent crystal itself is difficult and expensive to manufacture, and it has been difficult to apply it to the consumer field.

As described above, the isotropic diffraction grating has a utilization efficiency of about 50% on the outward path and about 20% on the return path.
Only about 10% of the round trip usage efficiency was obtained.

[0007] Further, since these are not only low in efficiency but also a diffraction grating having a symmetrical grating, the diffraction efficiency of +1 light and the diffraction efficiency of -1 order light are almost equal. When a plurality of photodetectors are provided and detection is performed by both, it is advantageous that both diffraction efficiencies are substantially equal, but it is not necessarily advantageous in the case of one photodetector.

That is, it has been proposed to use a single photodetector in order to reduce the size of the optical head. In this case, the fact that light is diffracted by the same amount of light in two directions means that the light It is not advantageous from the viewpoint of utilization efficiency. Of the light diffracted in two directions, it is advantageous to increase the amount of light diffracted toward the side where the photodetector is provided. For this reason, it has been desired to increase one diffracted light more than the other.

[0009]

For this purpose, it is necessary to use a diffraction grating in which the lateral direction of the unevenness of the diffraction grating is left-right asymmetric. However, this asymmetrical diffraction grating was originally a diffraction grating having a considerably fine pitch, and was extremely difficult to manufacture. For this reason, a method of manufacturing an asymmetrical diffraction grating with high productivity has been desired.

An object of the present invention is to solve such a problem and to provide a manufacturing method for easily manufacturing an optical diffraction grating having high utilization efficiency of one of a plurality of diffracted lights.

[0011]

According to the present invention, there is provided a method of manufacturing an optical diffraction grating in which a lattice-like convex portion is formed on a surface of a substrate, wherein a photoresist is formed on the surface of the substrate, and the photoresist is obliquely exposed. Then, development is performed so that the lateral side surfaces of the remaining photoresist lattice are asymmetrical, and then dry etching is performed, so that the lateral side surfaces of the lattice of the protrusions remaining by etching are asymmetrical. There is provided a method for manufacturing an optical diffraction grating characterized by the above.

[0012] Further, the above-mentioned optical diffraction method is characterized in that a substrate provided with a transparent material film is used as the substrate, and the lateral sides of the lattice of the projections of the transparent material film remaining by etching are asymmetric. A method for manufacturing a lattice, and a substrate formed so that the side surfaces of a convex portion are asymmetrical is used as a first substrate, and an optically anisotropic material is filled between the first substrate and the second substrate. A method for manufacturing the optical diffraction grating is provided.

Further, an optical head device using an optical diffraction grating manufactured by any of these manufacturing methods,
And, the light emitted from the light source is an optical diffraction grating, a retardation plate,
The reflected light passes through the objective lens, the phase difference plate, and the optical diffraction grating in order, and is reflected by the optical diffraction grating. The optical head device is provided so as to reach a photodetector.

The optical diffraction grating manufactured by the present invention is suitable for an optical head device for writing and / or reading information by irradiating light from a light source onto an optical recording medium through the optical diffraction grating. Used for

In the present invention, when the resist pattern is exposed with a photomask, the resist pattern is exposed in an oblique direction to form a grid-shaped (striped) convex portion which is asymmetrical on the surface of the transparent substrate. Further, a liquid crystal having optical anisotropy is filled between two substrates by using the substrate having the convex portions formed in this manner. Thus, an optical diffraction grating having high one-sided diffraction efficiency can be easily manufactured.

In the present invention, the substrate itself may be directly etched to form a projection, but the etching depth,
From the viewpoint of distribution and end point detection, it is preferable to use a transparent material film formed on the substrate surface.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a photoresist is formed on the surface of a substrate itself or on the surface of a substrate provided with a transparent material film, and the photoresist is obliquely exposed and developed to form a grid of the remaining photoresist. Is made to be asymmetrical, and then dry-etched so that the lateral side of the lattice of the protrusions left by the etching becomes asymmetrical.

FIG. 2 shows main steps of manufacturing a conventional optical diffraction grating. (A) is a sectional view showing a state where the photoresist is exposed and developed, and (B) is a sectional view showing a state where the transparent material film is dry-etched using the same. 1
1 is a substrate, 12 is a transparent material film, 13 is a photoresist,
Reference numeral 14 denotes a transparent material film after etching.

When ordinary exposure, development, and etching are performed as described above, the transparent material film has substantially the same inclination angle on the left and right sides of the short-side (left-right direction in the figure) convex portion of the lattice stripe. Become symmetric. In the optical diffraction grating having such a shape, the amount of diffracted light is substantially equal between the + 1st order and the -1st order. This is suitable when a plurality of light detectors are provided on the left and right to detect reflected light from an optical recording medium.

However, when only one of the photodetectors is provided, it is advantageous to diffract more light only on the side where the photodetector is provided. Although a photodetector is provided for both of them, the purpose is different, and the same applies to a case where a larger amount of light is required for one of the photodetectors. For this reason, the optical head device using the optical diffraction grating manufactured by the manufacturing method of the present invention is not limited to a single optical detector.

FIG. 1 shows the main steps of manufacturing the optical diffraction grating of the present invention, and shows an example in which a transparent material film is formed on the substrate surface. (A) is a cross-sectional view showing a state where a photoresist is exposed and developed, and (B) is a cross-sectional view showing a state where a transparent material film is dry-etched using the photoresist. Reference numeral 1 denotes a substrate, 2 denotes a transparent material film, 3 denotes a photoresist, and 4 denotes a transparent material film after etching.

When the normal exposure, development and etching are performed as described above, the transparent material film has different inclination angles on the left and right sides of the protrusions in the short direction (left and right directions in the drawing) of the lattice stripe, and the right and left are asymmetric. become. In an optical diffraction grating having such a shape, the amount of diffracted light differs between the + 1st order and the -1st order. This is preferable in the case where one light detector is provided on either the left or right to detect the reflected light from the optical recording medium, because the light amount can be effectively used.

The substrate used in the present invention is a transparent substrate such as glass. This transparent substrate may be formed a convex portion of the substrate surface is etched by using as it is, as in the example of this drawing, on a transparent substrate, SiON, a transparent material film such as SiO _2, reactive sputtering Method, evaporation method, plasma CV
It is preferable to form and use by D method or the like.

When an optically anisotropic material such as a liquid crystal is filled and not used, the refractive index of the transparent material film is adjusted to reduce the reflection at the interface between the transparent material film and the substrate. It is preferable to make the refractive index substantially equal to the refractive index.

When an optically anisotropic material such as liquid crystal is filled and used, the refractive index of the transparent material film is preferably substantially equal to the ordinary or extraordinary refractive index of liquid crystal or the like. Further, in order to reduce the reflection at the interface with the substrate, it is preferable that the refractive index of the transparent material film is made substantially equal to the refractive index of the substrate.

A photoresist is laminated on the transparent material film 2 by spin coating or the like, and the photoresist is irradiated with light through a photomask. At this time, by irradiating the light in an oblique direction and then developing, the left and right side surfaces of the protrusion of the remaining photoresist 3 in the short direction (left and right direction in the drawing) of the stripe have different inclination angles,
Left and right become asymmetric. In this example, the left side is substantially vertical, and the right side is inclined.

The light irradiation from the oblique direction is performed by irradiating light from the oblique direction. The angle of the oblique irradiation may be determined experimentally so as to obtain a desired pattern, but may be approximately 10 to 60 ° inclined from the vertical direction. Specifically, for example, the method may be performed as shown in FIG. FIG. 3 is a side view showing a light irradiation step from an oblique direction.

Since normal exposure is performed from the vertical direction, an exposure apparatus having the same structure as a light source is used, a prism is inserted into the optical path, and the optical path is bent obliquely.
In FIG. 3, 21 is a substrate, 22 is a transparent material film, 23 is a photoresist, 25 is a mask, 26 is a prism, 27
A is the incident light in the vertical direction, 27B is the light in the prism, 27
C indicates the traveling direction of light after passing through the prism.

Exposure from oblique directions is a typical exposure method, but is not limited to this. For example, light may be emitted from a diagonal direction using a known optical component such as a prism array structure in which a plurality of prisms are provided side by side or a method using mirror reflection.

Next, dry etching of the substrate itself or the transparent material film is performed using the asymmetric photoresist. This dry etching may be performed using a material which can etch the substrate itself or a transparent material film. At this time, by using a material that etches the transparent material film but hardly etches the substrate, the etching can be easily controlled, which is preferable in terms of the etching depth and the like.

As a result, an asymmetric convex portion of the transparent material film 4 as shown in FIG. 1B can be formed. The convex portion of the transparent material film 4 has a shape substantially similar to the shape of the photoresist 3. In this example, the left side of the projection of the transparent material film 4 is substantially vertical, and the right side is inclined.

The substrate provided with the asymmetrical convex lattice is
It can be used as an optical diffraction grating formed as such. An optically anisotropic material such as a liquid crystal can be filled between a substrate provided with the asymmetrical convex lattice and another substrate to be used as an optical diffraction grating. Also, in the above example, an example using a substrate provided with a transparent material film has been mainly described,
A substrate in which a projection is formed by etching the substrate itself can also be used.

FIG. 4 is a sectional view of an example of an optical diffraction grating using a liquid crystal. In FIG. 4, 31 is a substrate, 34 is a transparent material film having an asymmetrical convex portion, 35 is another substrate, 36 is a sealing material for sealing the periphery of two substrates, and 37 is a liquid crystal.

When this liquid crystal is used, it is advantageous to align the substrate and align the liquid crystal in a specific direction.
For example, when a substrate that has been subjected to an alignment treatment so that the longitudinal direction of the liquid crystal molecules is in the front-rear direction of FIG. 4 is used and one of the refractive indices of the liquid crystal and the refractive index of the transparent material film match, a high diffraction efficiency is obtained. Obtained and preferred.

This liquid crystal alignment treatment is performed by oblique deposition of an inorganic film such as SiO, polyimide, polyamide, SiO 2 or the like.
_A rubbing treatment may be performed by forming a _second- class film. Further, an electrode may be formed as necessary, and the function of the optical diffraction grating may be changed by changing the alignment state of some liquid crystals by applying a voltage. More specifically, the diffraction ability of the optical diffraction grating, the aperture ratio, the optical rotation, and the focal length may be changed depending on the voltage application state.

The optical diffraction grating having the asymmetrical convex grating is suitably used for an optical head device. In an optical head device, this optical diffraction grating is used by being arranged between a light source and an optical recording medium. As a specific configuration,
There is a configuration shown in FIG. In FIG. 5, 41 is a light source such as a laser diode, 42 is an optical diffraction grating, and 43 is 1/4.
A retardation plate such as a λ plate, 44 is an objective lens, 45 is an optical recording medium, 46 is a photodetector, and 47A and 47B are diffracted lights.

In this example, since the quantity of light of the diffracted light 47A is larger than that of the diffracted light 47A due to the asymmetric optical diffraction grating, the photodetector is provided only on the diffracted light 47B side. Since the photodetector is provided only on one side, the light amount only needs to be large on that side. For this reason, since the light amount of the light source 41 is small, there is an advantage that the optical head device is reduced in size and power consumption is reduced.

The example of the optical head device is only a typical example, and the present invention is not limited to this example. For example, by adding a prism or mirror to this, the optical path can be bent, the objective lens can be exchanged, the focal length can be changed with a liquid crystal lens, etc., so that optical recording media with different focal positions can be read and written. Is also good. Further, a separate diffraction grating may be added to divide the light beam into three beams.

[0039]

【Example】

In Example 1 on a glass substrate having a refractive index of 1.52, it was formed by vacuum deposition of SiO ₂ film having a refractive index of 1.44 to a thickness of 600nm as a transparent material film. A ₂ μm-thick positive photoresist was applied on the SiO ₂ film by spin coating, and prebaked. After that, normal adhesion /
Using a proximity exposure apparatus, contact exposure was performed using a mask pattern of 4 μm pitch.

At this time, a refractive index of about 1.
By placing the right angle prism made of acrylic resin of No. 5 on the mask, the ultraviolet light vertically incident from above by about 25 °
The photoresist was exposed at an incident angle of. Thereafter, development and post-baking were performed to form photoresist patterns having asymmetric shapes in which the rising angles of the photoresist were about 85 ° and about 60 °.

Thereafter, dry etching is performed by reactive ion etching (RIE) using a mixed gas of C ₂ F ₆ and O _{2 so} that an asymmetric shape of the photoresist is formed on the SiO ₂ film. . Thereafter, the remaining photoresist was removed to realize an optical diffraction grating having an asymmetric SiO ₂ film convex portion.

The obtained optical diffraction grating has a wavelength of 650 nm.
About 12% for the + 1st order and about 1 for the -1st order
% Asymmetry ratio of about 10 and transmittance of about 50%.

Example 2 An SiON film having a refractive index of 1.52 was formed to a thickness of 1.8 μm by a plasma CVD (p-CVD) method on a glass substrate having a refractive index of 1.52. S
A 2.5 μm-thick positive photoresist was applied on the iON film by spin coating, and prebaked. Thereafter, contact exposure was performed using a mask pattern of 4 μm pitch.

At this time, a refractive index of 1.8 was formed on the photomask.
By arranging the glass right-angle prism No. 5 on the mask, the photoresist was exposed to ultraviolet light vertically incident from above at an incident angle of about 60 °. Thereafter, development and post-baking were performed to form photoresist patterns having asymmetric shapes in which the rising angles of the photoresist were about 85 ° and about 60 °.

Thereafter, dry etching was performed by RIE using a mixed gas of C ₂ F ₆ and O _{2 so} that an asymmetric shape of the photoresist was formed on the SiON film. After coating the anti-reflection film on the substrate from which the remaining photoresist was removed and the opposite glass substrate, polyimide was applied and baked to form an alignment film.

After the rubbing alignment treatment, these substrates were
The periphery was thermocompression-bonded with a sealing material including a μm spacer, and a nematic liquid crystal having an ordinary light refractive index of 1.52 and an extraordinary light refractive index of 1.77 was filled and the injection port was sealed.

The fabricated polarization dependent diffraction grating has a wavelength of 65
With respect to the semiconductor laser light of 0 nm, the characteristics showed an asymmetry ratio of about 3.3, that is, about + 50% of the + 1st order and about 15% of the −1st order with respect to the S-polarized semiconductor laser light. In addition, it showed a transmittance of 95% with respect to the P-polarized semiconductor laser light.

When this optical diffraction grating is used by being incorporated in the optical head device shown in FIG. 5, light on the outward path from the light source passes through the optical diffraction grating with high efficiency as it is, and light on the return path is diffracted by the optical diffraction grating. In this case, diffracted light having a large amount of light toward one of the photodetectors was obtained.

[0049]

According to the present invention, an optical diffraction grating having an asymmetric structure can be manufactured with high productivity. In particular, since it is only necessary to diagonally expose light, the existing manufacturing apparatus can be easily diverted to manufacture. Therefore, the production can be easily performed with a slight change of the existing production line such as addition of a prism, and the co-production with the conventional optical diffraction grating having a symmetric structure is easy, so that the productivity is extremely high.

The optical diffraction grating having this asymmetric structure can increase the efficiency of using one of a plurality of diffracted lights, and is useful for reducing the size and power consumption of an optical head device. The present invention can be applied to various applications within a range that does not impair the effects of the present invention.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a manufacturing process of an optical diffraction grating of the present invention.

FIG. 2 is a cross-sectional view showing a manufacturing process of a conventional optical diffraction grating having a symmetric structure.

FIG. 3 is a side view showing a light irradiation step from an oblique direction according to the present invention.

FIG. 4 is a cross-sectional view of an example of an optical diffraction grating using the liquid crystal of the present invention.

FIG. 5 is a schematic view of a typical example of an optical head device using the optical diffraction grating of the present invention.

[Explanation of symbols]

 1: substrate 2: transparent material film 3: photoresist 4: transparent material film

Claims (5)

[Claims] In a method of manufacturing an optical diffraction grating having a lattice-like convex portion formed on a surface of a substrate, a photoresist is formed on a surface of the substrate, and the photoresist is obliquely exposed and developed to be left. The lateral sides of the lattice of the photoresist are made asymmetrical, and then dry etching is performed so that the lateral sides of the lattice of the protrusions left by the etching are made asymmetrical. Manufacturing method of optical diffraction grating. 2. The method according to claim 1, wherein a substrate provided with a transparent material film is used as the substrate, and the lateral sides of the lattice of the projections of the transparent material film remaining by etching are asymmetrical. Of manufacturing an optical diffraction grating. 3. The method according to claim 1, wherein the first substrate is a substrate formed such that the side surfaces of the projections are asymmetrical, and an optically anisotropic material is filled between the first substrate and the second substrate. The method for manufacturing an optical diffraction grating according to claim 1 or 2, wherein 4. An optical head device using an optical diffraction grating manufactured by the method according to claim 1, 2 or 3. 5. A light emitted from a light source passes through an optical diffraction grating, a phase difference plate, and an objective lens in order, and is applied to an optical recording medium. The reflected light passes through the objective lens, the phase difference plate, and the optical diffraction grating in order. 5. The optical head device according to claim 4, wherein the reflected light is diffracted by an optical diffraction grating and reaches a photodetector located at a position different from a light source.