Description

DIFFRACTION GRATING AND METHOD OF FABRICATION

THEREOF

Technical Field

[1] The present invention relates to a diffraction grating and a method of fabrication thereof, and more particularly to a diffraction grating for an optical pickup with anti- reflection coating layers formed when a grating pattern on a glass or plastic substrate, and a method for manufacturing the diffraction grating. Background Art

[2] Generally, an optical pickup device is used in a CD player or DVD player adapted to read data stored in an optical disc such as a CD or a DVD. Such an optical pickup device includes a diffraction element or phase element for detecting data in accordance with detection of an optical signal, in order to read out information from an optical recording medium.

[3] In detail, such an optical pickup device includes a laser diode for emitting laser beam, a diffraction grating for distributing the amount of the laser beam emitted from the laser diode in accordance with a certain ratio, a beam splitter for transmitting or reflecting a beam reflected from an optical disc based on information recorded on the optical disc and an incident beam from the diffraction grating in accordance with a certain ratio, and a photodetector for receiving a beam from the beam splitter, and detecting data based on the received beam.

[4] The diffraction grating conventionally used in the above-mentioned optical pickup device has a structure in which a grating pattern consisting of uniformly-spaced protrusions and grooves is formed on a glass substrate.

[5] The diffraction grating functions to split a laser beam emitted from the laser diode into a main beam and two sub-beams which are, in turn, directed to an optical disk.

[6] Conventionally, the conventional diffraction grating is manufactured by forming an oxide thin film over a glass substrate made of bulk glass or quartz in accordance with a sputtering, spin coating, or spin-on-glass method, and subjecting the oxide thin film to a photolithography process using an etching solution or etching gas.

[7] However, it is difficult to achieve mass production of the diffraction grating using the above-mentioned conventional manufacturing method because it is difficult to achieve a desired uniformity of the grating pattern formed in accordance with the manufacturing method, and thus to obtain a stable performance of the diffraction grating.

[8] For this reason, an improved manufacturing method as disclosed in Korean Patent

Unexamined Publication No. 2005-17079 has been proposed. This method involves

coating a thin layer of an ultraviolet-setting polymer over a glass substrate as a pattern layer of a diffraction grating, pressing a stamper, formed with a shaping pattern corresponding to a grating pattern to be formed, against the thin layer, and irradiating ultraviolet rays to the thin layer, thereby curing the thin layer.

[9] In order to enhance the transmittance of incident light in the form of an optical beam, an anti-reflection (AR) coating treatment is conducted after the grating pattern, which has a structure of protrusions and grooves, is formed on the glass substrate using a polymer or organic material in accordance with an ultraviolet (UV) or embossing method. The anti-reflection coating treatment is conducted on the substrate and the grating pattern, using TiO , SiO , etc.

[10] FIG. 1 is a sectional view illustrating a diffraction grating with a grating pattern made of a polymer or organic material and subjected to an anti-reflection coating treatment.

[11] As shown in FIG. 1, a thin layer 12 made of a polymer or organic material is formed over a planar and transparent glass substrate 11, as a grating pattern layer. Anti-reflection coating layers 13 and 14 are formed on protrusions and grooves of the thin layer 12, respectively. Also, an anti-reflection coating layer 15 is formed over the surface of the glass substrate 11 facing a laser diode to receive a laser beam emitted from the laser diode.

[12] The formation of each anti-reflection coating layer is achieved by alternately depositing TiO and SiO in a chamber maintained in a vacuum state (10 Torr or less) at a high temperature (100 to 300 ⁰ C).

[13] Since the polymer or organic material of the thin layer 12 formed with the grating pattern is subjected to a high temperature in a vacuum state, it exhibits insufficient thermal stability. As a result, the polymer or organic material of the thin layer 12 expands.

[14] The polymer or organic material exhibits an expansion degree depending on the curing degree thereof.

[15] On the other hand, when the anti-reflection coating process is carried out at a low temperature (40 to 100 ⁰ C) in a vacuum state, there is a problem in that the coating layers deposited on the protrusions and grooves have no reliability due to insufficient thermal stability thereof.

[16] As a result, the optical characteristics of the diffraction grating vary during every anti-reflection coating process. In association with the anti-reflection coating process for the protrusions and grooves, there are problems of lack of reliability at a low temperature and a degradation in yield at a high temperature. Disclosure of Invention

Technical Problem

[17] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a diffraction grating and a method for manufacturing the same which are capable of achieving mass production of diffraction gratings having desired optical characteristics and reliability by conducting an anti- reflection coating process for glass substrates, which exhibit superior thermal stability and reliability over those of a polymer or organic material of a thin layer to form a grating pattern, before the formation of the thin layer, and then bonding the glass substrates. Technical Solution

[18] In accordance with an aspect, the present invention provides a diffraction grating comprising a glass substrate and a grating pattern formed on the glass substrate, wherein: the glass substrate comprises a first glass substrate formed with an anti- reflection coating layer and a second glass substrate formed with an anti-reflection coating layer in advance; a thin layer made of a polymer or organic material is coated over the first glass substrate after the formation of the anti-reflection coating layer on the first glass substrate, to form protrusions/grooves constituting the grating pattern; and the second glass substrate is bonded to the protrusions/grooves of the first glass substrate via an adhesive layer made of a polymer or organic material having a refractive index different from that of the thin layer and cured by ultraviolet irradiation.

[19] The anti-reflection coating layers may contain TiO and SiO .

[20] The polymer or organic material may be selected from polycarbonate, polyimide, polyalkylate, polyester sulfone, polyolefin, acrylate, polyether imide, acryl, epoxy, urethane, or polyester.

[21] A phase delay plate may be interposed between the protrusions/grooves and the second glass substrate. In this case, the adhesive layer may comprise adhesive layers respectively formed on opposite surfaces of the phase delay plate.

[22] The phase delay plate may comprise two or more phase delay plates.

[23] In accordance with another aspect, the present invention provides a method for manufacturing a diffraction grating, comprising: forming anti-reflection coating layers over one surface of a first glass substrate and one surface of a second glass substrate; coating a thin layer made of a polymer or organic material over the other surface of the first glass substrate; pressing a stamper formed with a grating pattern onto the thin layer, thereby forming protrusions/grooves at the thin layer; irradiating ultraviolet rays to the thin layer formed with the protrusions/grooves; bonding the first and second glass substrates via an adhesive layer made of a polymer or organic material having a refractive index different from that of the thin layer and applied between the other

surface of the second glass substrate and the protrusions/grooves; and irradiating ultraviolet rays to the adhesive layer, thereby curing the adhesive layer. [24] The method may further comprise interposing a phase delay plate between the protrusions/grooves and the second glass substrate while applying the adhesive layer to opposite surfaces of the phase delay plate.

Advantageous Effects

[25] In accordance with the present invention, an improvement in the reproducibility of the optical characteristics and reliability in an anti-reflection coating process is achieved, in order to improve the light transmittance of the above-described diffraction grating and phase-delaying diffraction grating. Accordingly, it is possible to maintain desired characteristics of the device while achieving an enhancement in yield, and thus a reduction in the manufacturing costs. Brief Description of the Drawings

[26] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[27] FIG. 1 is a sectional view illustrating a diffraction grating with a grating pattern made of a polymer or organic material and subjected to an anti-reflection coating treatment;

[28] FIG. 2 is a sectional view illustrating a diffraction grating subjected to an anti- reflection coating process in accordance with a first embodiment of the present invention;

[29] FIG. 3 is a sectional view illustrating a diffraction grating subjected to an anti- reflection coating process in accordance with a second embodiment of the present invention;

[30] FIG. 4 is a flow chart illustrating a method for manufacturing a diffraction grating with anti-reflection coating layers in accordance with the present invention; and

[31] FIG. 5 is a flow chart illustrating another method for manufacturing a diffraction grating with anti-reflection coating layers in accordance with the present invention.

[32] <Description of Reference Numerals>

[33] 11, 17: glass substrate 12: thin layer

[34] 13, 14, 15: anti-reflection coating layer 16, 18: adhesive layer

[35] 20: protrusions/grooves 30: phase delay plate

Best Mode for Carrying Out the Invention

[36] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the annexed drawings.

[37] FIG. 2 is a sectional view illustrating a diffraction grating subjected to an anti-

reflection coating process in accordance with a first embodiment of the present invention.

[38] Referring to FIG. 2, the diffraction grating according to the first embodiment of the present invention includes a first glass substrate 11 formed with an anti-reflection coating layer 14 at one surface of the first glass substrate 11, and formed with a thin layer 12 at the other surface of the first glass substrate 11. The thin layer 12 is made of a polymer or organic material, and is formed with protrusions/grooves 20 constituting a grating pattern. The diffraction grating also includes a second glass substrate 17 formed with an anti-reflection coating layer 15 at one surface of the second glass substrate 17, and an adhesive layer 16 interposed between the protrusions/grooves 20 of the thin layer 12 and the second glass substrate 17.

[39] The formation of the thin layer 12 is achieved by coating an ultraviolet (UV)-setting polymer or organic material over the glass substrate 11. The formation of the grating pattern consisting of the protrusions/grooves 20 is achieved by pressing the thin layer 12 by a stamper formed with a shaping pattern corresponding to the grating pattern.

[40] The anti-reflection coating layers 14 and 15 are formed to increase the light transmittance of the diffraction grating. The formation of the anti-reflection coating layers 14 and 15 is achieved by loading the glass substrates 11 and 17 in a chamber maintained in a vacuum state (10 Torr or less), and then alternately depositing TiO and SiO over the glass substrates 11 and 17 at a high temperature (100 to 300 ⁰ C).

[41] The formation of the anti-reflection coating layers 14 and 15 is carried out earlier than the formation of the thin layer 12 and adhesive layer 16 over the glass substrates 11 and 17.

[42] When the anti-reflection coating layers 14 and 15 are formed over the glass substrates 11 and 17, respectively, before the formation of the thin layer 12, as described above, it is possible to prevent a variation in the light transmittance of the diffraction grating depending on the uniformity of the deposition equipment or the deposition height. If such an anti-reflection coating layers is formed over a glass substrate after formation of the grating pattern of the thin layer made of a polymer or organic material over the glass substrate, the polymer may expand by several nanometers due to the coating process carried out in a vacuum state at a high temperature of about 200 ⁰ C, so that the above-described variation may occur when TiO and SiO are deposited to a height of about 300 to 800nm, even though the diffraction grating has a uniform light transmittance before the formation of the anti-reflection coating layers.

[43] Meanwhile, the adhesive layer 16 is made of a polymer or organic material having a refractive index different from that of the polymer or organic material of the thin layer 12.

[44] The polymer or organic material used for the thin layer 12 or adhesive layer 16 may be selected from polycarbonate, polyimide, polyalkylate, polyester sulfone, polyolefin, acrylate, polyether imide, acryl, epoxy, urethane, or polyester.

[45] The polymer or organic material coated over the glass substrate 11 as the thin layer

12 is inexpensive, and exhibits superior light transmittance, shapability, and curability.

[46] In particular, a UV-setting polymer may be coated over the glass substrate 11, using a spin collating or dropping method. The coating may also be achieved using a hot embossing method.

[47] The adhesive layer 16 is applied between the protrusions/grooves 20 of the first glass substrate 11 and the second glass substrate 17, and is then cured in accordance with UV irradiation. Thus, the diffraction grating according to the first embodiment of the present invention has a structure in which the first and second glass substrates 11 and 17 are bonded to each other via two polymer or organic material layers interposed between the first and second glass substrates 11 and 17.

[48] FIG. 3 is a sectional view illustrating a diffraction grating subjected to an anti- reflection coating process in accordance with a second embodiment of the present invention.

[49] Referring to FIG. 3, the diffraction grating according to the second embodiment of the present invention includes a first glass substrate 11 formed with an anti-reflection coating layer 14 at one surface of the first glass substrate 11, and formed with a thin layer 12 at the other surface of the first glass substrate 11. The thin layer 12 is made of a polymer or organic material, and is formed with protrusions/grooves 20 constituting a grating pattern. The diffraction grating also includes a second glass substrate 17 formed with an anti-reflection coating layer 15 at one surface of the second glass substrate 17, a phase delay plate 30 arranged between the protrusions/grooves 20 of the thin layer 12, an adhesive layer 16 interposed between the protrusions/grooves 20 and the phase delay plate 30, and an adhesive layer 18 interposed between the second glass substrate 17 and the phase delay plate 30.

[50] As in the first embodiment, the adhesive layers 16 and 18 of the second embodiment are made of a polymer or organic material having a refractive index different from that of the polymer or organic material of the thin layer 12.

[51] In the second embodiment, the adhesive layers 16 and 18 are applied between the protrusions/grooves 20 of the first glass substrate 11 and the phase delay plate 30 and between the phase delay plate 30 and the second glass substrate 17, respectively, and are then cured in accordance with UV irradiation. Thus, the diffraction grating according to the second embodiment of the present invention has a structure in which the first and second glass substrates 11 and 17 are bonded to each other via three polymer or organic material layers and the phase delay plate 30 interposed between the

first and second glass substrates 11 and 17.

[52] The phase delay plate 30, which is a phase-delaying diffraction grating, may be arranged between the glass substrates 11 and 17 subjected to an anti-reflection coating process under the condition in which no anti-reflection coating layer is formed over the protrusions/grooves 20 of the thin layer 13, and may then be bonded to the glass substrates 11 and 17. In this case, a more stable diffraction grating can be obtained.

[53] Two or more phase delay plates 30 may be arranged between the glass substrates 11 and 17. In this case, the grating pattern may be formed on the uppermost or lowermost phase delay plate 30. Thus, the present invention can be applied to all phase-delaying diffraction gratings.

[54] FIG. 4 is a flow chart illustrating a method for manufacturing a diffraction grating with anti-reflection coating layers in accordance with the present invention.

[55] In accordance with a method for manufacturing the diffraction grating with anti- reflection coating layers according to the first embodiment of the present invention, the anti-reflection coating layers 14 and 15 are first formed over one surface of the first glass substrate 11 and one surface of the second glass substrate 17 by an anti-reflection coating process (S41), as shown in FIG. 4.

[56] Thereafter, the thin layer 12 is formed over the surface of the first glass substrate 11 opposite to the surface formed with the anti-reflection coating layer 14, using a polymer or organic material (S42).

[57] Subsequently, a stamper formed with a shaping pattern corresponding to a grating pattern to be formed is pressed onto the thin layer 12, to form the protrusions/grooves 20 of the grating pattern (S43).

[58] The stamper is fabricated by fabricating a master having a pattern corresponding to the grating pattern in accordance with a photolithography method, and then plating a metal over the surface of the pattern formed at the master, thereby forming a shaping pattern corresponding to the grating pattern.

[59] The master fabrication process may be achieved using various methods such as a photolithography method, a spin-on-glass (SOG) method, or an etching method.

[60] When the photolithography method is used, photoresist is first spin-coated over a glass substrate or a wafer, using a general semiconductor process.

[61] The thickness of the coating is similar to the thickness of the final product. The coating is formed to have a considerably small thickness of 100 to 400nm.

[62] After the coating process, the wafer or glass substrate is baked at a temperature of

70 to 12O ⁰ C on a plate or in an oven. A photomask is then laid on the wafer or glass substrate which is, in turn, subjected to UV irradiation. After the UV irradiation, the wafer or glass substrate is dipped in a developing solution. Thus, a master having a desired pattern is obtained. Although the above process is a general semiconductor

process, it is characterized in that the thickness of the master is very small. [63] In order to cure the thin layer 12 formed with the protrusions/grooves, a process for irradiating UV rays to the thin layer 12 is carried out (S44). [64] The formation of the protrusions/grooves 20 at the thin layer 12 as the grating pattern may be achieved using a hot embossing method other than the UV curing method. [65] Thereafter, the adhesive layer 16 made of a polymer or organic material having a refractive index different from that of the thin layer 12 is applied between the other surface of the second glass substrate 17 and the protrusions/grooves 20, to bond the first and second glass substrates 11 and 17(S45, S46). [66] Since the adhesive layer 16 is made of a liquid-phase polymer or organic material,

UV rays are irradiated to the applied adhesive layer 16, to cure the adhesive layer

16(S47). Thus, the diffraction grating is obtained. [67] FIG. 5 is a flow chart illustrating another method for manufacturing a diffraction grating with anti-reflection coating layers in accordance with the present invention. [68] The method of FIG. 5 is adapted to manufacture the diffraction grating with anti- reflection coating layers according to the second embodiment of the present invention.

The diffraction grating of the second embodiment has a structure added with a phase delay plate as a phase-delaying diffraction grating. [69] In accordance with this method, the anti-reflection coating layers 14 and 15 are first formed over one surface of the first glass substrate 11 and one surface of the second glass substrate 17 by an anti-reflection coating process (S51). [70] Thereafter, the thin layer 12 is formed over the surface of the first glass substrate 11 opposite to the surface formed with the anti-reflection coating layer 14, using a polymer or organic material (S52). [71] Subsequently, a stamper formed with a shaping pattern corresponding to a grating pattern to be formed is pressed onto the thin layer 12, to form the protrusions/grooves

20 of the grating pattern (S53). In order to cure the thin layer 12 formed with the protrusions/grooves 20, a process for irradiating UV rays to the thin layer 12 is carried out (S54). [72] The phase delay plate 30 is then arranged between the protrusions/grooves 20 of the first glass substrate 11 and the other surface of the second glass substrate 17 (S55).

Subsequently, a polymer or organic material is applied to the upper and lower surfaces of the phase delay plate 30, to form the adhesive layers 16 and 18 between the protrusions/grooves 20 and the phase delay plate 30 and between the second glass substrate 17 and the phase delay plate 30 (S56). [73] The first and second glass substrates 11 and 17 are bonded via the liquid-phase adhesive layers 16 and 18 made of the polymer or organic material (S57). UV rays are

then irradiated to the adhesive layers 16 and 18 to cure the adhesive layers 16 and 18 (S58). Thus, the diffraction grating is obtained.

[74] Of course, two or more phase delay plates 30 may be interposed between the first and second glass substrates 11 and 17, if necessary.

[75] As apparent from the above description, in the diffraction grating of the present invention, the adhesive layers 16 and 18 are made of a polymer or organic material having a refractive index different from that of the protrusions/grooves 20. When appropriate polymer or organic materials having different refractive indexes are used in combination as the thin layer 12 and adhesive layers 16 and 18, the resultant diffraction grating can exhibit optical characteristics identical to those of a structure including only the thin layer 12 with the protrusions/grooves 20.

[76] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.