TREA

Semiconductor light emitting element, manufacturing method therefor, and compound semiconductor light emitting diode

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Information

  • Patent Application
  • 20070290216
  • Publication Number
    20070290216
  • Date Filed
    June 19, 2007
    17 years ago
  • Date Published
    December 20, 2007
    17 years ago
Information
Abstract
A semiconductor light emitting element is provided with a transparent substrate for improving the optical extraction efficiency by using a transparent substrate. The semiconductor light emitting element includes a main body constructed of an n-Al0.6Ga0.4As current diffusion layer, an n-Al0.5In0.5P cladding layer, an AlGaInP active layer, a p-Al0.5In0.5P cladding layer, a p-GaInP interlayer and a p-GaP contact layer. An n-GaP transparent substrate is placed under the main body. A p-GaP transparent substrate is placed on top of the main body. The n-GaP transparent substrate and the p-GaP transparent substrate have transparency with respect to light emitted from the AlGaInP light emitting layer.
Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:



FIG. 1 is a schematic sectional view of a conventional semiconductor light emitting element;



FIG. 2 is a schematic sectional view of another conventional semiconductor light emitting element;



FIG. 3 is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 4 is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 5 is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 6 is a graph showing results of comparing the light transmittance of a heavily doped GaP substrate and the light transmittance of a lightly doped GaP substrate;



FIG. 7 is a graph showing changes in the transmittance of the heavily doped GaP substrate and the lightly doped GaP substrate with respect to a change in the optical path length;



FIG. 8 is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 9 is a schematic view of an essential part of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 10 is a schematic view of an essential part of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 11 is a schematic view of an essential part of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 12 is a schematic view of an essential part of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 13 is a graph showing the chip size dependency on optimal thickness (height) value of a transparent substrate;



FIG. 14 is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 15 is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 16A is a schematic sectional view of a semiconductor light emitting element according to one embodiment of the present invention;



FIG. 16B is a schematic perspective view of the semiconductor light emitting element of FIG. 16A;



FIG. 17 is a schematic sectional view of a LED of a first embodiment of the present invention;



FIG. 18 is one process chart of the manufacturing method of the LED of the first embodiment;



FIG. 19 is a schematic sectional view of a jig used for manufacturing the LED of the first embodiment;



FIG. 20 is a schematic sectional view of the LED of a second embodiment of the present invention;



FIG. 21 is a schematic sectional view of the LED of a third embodiment of the present invention;



FIG. 22A is one process chart of the manufacturing method of the LED of the third embodiment;



FIG. 22B is one process chart of the manufacturing method of the LED of the third embodiment;



FIG. 22C is one process chart of the manufacturing method of the LED of the third embodiment;



FIG. 22D is one process chart of the manufacturing method of the LED of the third embodiment; and



FIG. 22E is one process chart of the manufacturing method of the LED of the third embodiment.





DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be given below in detail to semiconductor light emitting elements, a manufacturing method therefor and compound semiconductor light emitting diodes of the present invention in embodiments with reference to drawings.


First Embodiment


FIG. 17 shows a schematic sectional view of a LED according to a first embodiment of the present invention.


The LED has a main body 1750, an n-GaP transparent substrate 1701 placed under the main body 1750, and a p-GaP transparent substrate 1708 placed on top of the main body 1750. It is noted that the n-GaP transparent substrate 1701 is one example of the first transparent substrate, and that the p-GaP transparent substrate 1708 is one example of the second transparent substrate.


The main body 1750 is constructed of an n-Al0.6Ga0.4As current diffusion layer 1702, an n-Al0.5In0.5P cladding layer 1703, an AlGaInP active layer 1704, a p-Al0.5In0.5P cladding layer 1705, a p-GaInP interlayer 1706, and a p-GaP contact layer 1707. It is noted that the AlGaInP light emitting layer 1705 is one example of the light emitting layer. Moreover, the n-Al0.6Ga0.4As current diffusion layer 1702 and the n-Al0.5In0.5P cladding layer 1703 constitute examples of a first conductive type semiconductor layer. The p-Al0.5In0.5P cladding layer 1705, the p-GaInP interlayer 1706 and the p-GaP contact layer 1707 constitute examples of a second conductive type semiconductor layer.


The AlGaInP light emitting layer 1705 is a quaternary system light emitting layer that emits light of an emission wavelength of a red color. The n-GaP transparent substrate 1701 and the p-GaP transparent substrate 1708 have transparency with respect to light emitted from the AlGaInP light emitting layer 1705.


An electrode 1709 is formed beneath the n-GaP transparent substrate 1701. An electrode 1710 is formed on the p-GaP transparent substrate 1708.


The manufacturing method of the LED is described below.


First of all, as shown in FIG. 18, an n-GaAs buffer layer 1802, the n-Al0.6Ga0.4As current diffusion layer 1702, the n-Al0.5In0.5P cladding layer 1703, the AlGaInP active layer 1704, the p-Al0.5In0.5P cladding layer 1705, the p-GaInP interlayer 1706 and the p-GaP contact layer 1707 are layered in this order by using the MOCVD method on an n-GaAs substrate 1801 as one example of the first conductive type semiconductor substrate, so that a LED structure wafer 1850 is formed.


The AlGaInP active layer 1704 has a quantum well structure. More in detail, the AlGaInP active layer 1704 is formed by alternately layering an (Al0.05Ga0.95)0.5In0.5p well layer and an (Al0.5Ga0.5)0.5In0.5P barrier layer. Then, eight pairs of the (Al0.05Ga0.95)0.5In0.5P well layer and the (Al0.5Ga0.5)0.5In0.5P barrier layer are provided.


The substrate and the layers have thickness dimensions provided as: 250 μm of the n-GaAs substrate 1801; 1.0 μm of the GaAs buffer layer 1802; 5.0 μm of the n-Al0.6Ga0.4As current diffusion layer 1702; 1.0 μm of the n-Al0.5In0.5P cladding layer 1703; 0.5 μm of the AlGaInP active layer 1704; 1.0 μm of the p-Al0.5In0.5P cladding layer 1705; 1.0 μm of the p-GaInP interlayer 1706; and 4.0 μm of the p-GaP contact layer 1707.


In the substrate or the layers, Te is used as an n-type dopant, while Mg is used as a p-type dopant.


The substrate and the layers have carrier densities provided as: 1.0×1018 cm−3 of the n-GaAs substrate 1801; 5×1017 cm−3 of the n-GaAs buffer layer 1802; 1.0×1018 cm−3 of the n-Al0.6Ga0.4As current diffusion layer 1702; 5×1017 cm−3 of the n-Al0.5In0.5P cladding layer 1703; nondoped AlGaInP active layer 1704; 5×1017 cm−3 of the p-Al0.5In0.5P cladding layer 1705; 1.0×1018 cm−3 of the p-GaInP interlayer 1706; and 2.0×1018 cm−3 of the p-GaP contact layer 1707.


Next, a half dicing groove is formed by half dicing at a prescribed pitch on the epitaxial surface of the wafer 20. At this time, a depth of about 10 to 50 μm is appropriate for the half dicing groove in the point that the strength of the LED structure wafer is maintained.


Next, the p-GaP transparent substrate 1708 is bonded directly to the epitaxial surface (upper surface of the p-GaP contact layer 1707) of the LED structure wafer 1850 by using an affixing jig 1950 made of quartz as shown in FIG. 19. The carrier density of the p-GaP transparent substrate 1708 is set at 5.0×1017 cm−3.


The jig 1950 has a first quartz plate 1951 that supports the wafer, a second quartz plate 1952 located on top of the first quartz plate 1951, and a pressurizing section 1953 that receives a force of a prescribed magnitude to pressurize the second quartz plate 1952.


The pressurizing section 1953 is guided in the vertical direction by a frame 1954 that has a roughly bracket-like shape when viewed from the front. The frame 1954 is engaged with the first quartz plate 1951, so that a force is appropriately transferred to the second quartz plate 1952 located between the first quartz plate 1951 and the pressurizing section 1953.


A carbon sheet 1955 is placed between the first quartz plate 1951 and the LED structure wafer 1850. A carbon sheet 1956 and a PBN (pyrolytic boron nitride) plate 1957 are placed between the second quartz plate 1952 and the p-GaP transparent substrate 1708.


By using the jig 1950 described above, the p-GaP transparent substrate 1708 is brought in contact with the p-GaP contact layer 1707, and then a force of, for example, 0.3 to 0.8 N·m is applied to the pressurizing section 1953 so as to make a compression force take effect on the contact plane of the p-GaP transparent substrate 1708 and the p-GaP contact layer 1707. The jig 1950 in the state is set in a heating furnace and heated for about 30 minutes at a temperature of about 800° C. under a hydrogen atmosphere. Then, the p-GaP transparent substrate 1708 is bonded directly to the LED structure wafer 1850.


Next, the LED structure wafer 1850 and the p-GaP transparent substrate 1708 are cooled down, and thereafter the jig 1950 is taken from the heating furnace. The n-GaAs substrate 1801 and the n-GaAs buffer layer 1802 are removed by dissolution with a liquid mixture of ammonia water, oxygenated water and water.


Next, the n-GaP transparent substrate 1701 is bonded directly to the surface (AlGaAs surface) exposed by removing the n-GaAs substrate 1801 and the n-GaAs buffer layer 1802. The bonding of the n-GaP transparent substrates 1701 is performed by processing under pressure and heating as in the case of the p-GaP transparent substrate 1708. Moreover, the carrier density of the n-GaP transparent substrate 1701 is set at 5.0×1017 cm−3.


Subsequently, electrode forming and chip-making process, which belong to the general manufacturing method of a semiconductor light emitting element, are carried out. Thereby, a high-intensity red LED of an emission wavelength of 640 nm as shown in FIG. 17 is completed.


According to the above-stated LED, the n-GaP transparent substrate 1701, which has transparency with respect to light emitted from the AlGaInP active layer 1704, is placed under the main body 1750. The p-GaP transparent substrate 1708, which has transparency with respect to the light emitted from the AlGaInP active layer 1704, is placed on top of the main body 1750. Thereby, light can efficiently be taken to the outside via the n-GaP transparent substrate 1701 and the p-GaP transparent substrate 1708. That is, the optical extraction efficiency can be improved.


In the present embodiment, an electrode material of AuSi/Au is selected as the material of the electrode 1709, and AuBe/Au is selected as the material of the electrode 1710. That is, in the present embodiment, the electrodes 1709 and 1710 are obtained by processing the AuSi/Au layer and the AuBe/Au layer into arbitrary shapes by the photolithography method and wet etching.


Moreover, after forming the electrodes 1709 and 1710, half dicing is carried out for separation into a prescribed chip size. At this time, by selecting a bevelable dicing blade, the side surface of the element can easily be processed into a slope shape. As a result, the side surface of one of the n-GaP transparent substrate 1701 and the p-GaP transparent substrate 1708 is allowed to have a slope shape.


The process with the bevelable dicing blade is carried out on a surface opposite from the surface that has previously undergone half dicing, this time. Thereby, the other side surface of the n-GaP transparent substrate 1701 and the p-GaP transparent substrate 1708 is allowed to have a slope shape.


The materials and techniques selected as above are not limited, and all sorts of materials and techniques of, for example, wet etching and dry etching can be selected. However, the technique of dicing seems to be appropriate in the point that it does not select (depend on) the material.


The manufacturing process of the present embodiment is applied to not only LED having the quaternary system light emitting layer made of AlGaInP but also any of the light emitting layer formed of semiconductor crystals.


Second Embodiment


FIG. 20 shows a schematic sectional view of a LED according to a second embodiment of the present invention. In FIG. 20, the same components as those of the LED of the first embodiment shown in FIG. 17 are denoted by same reference numerals as those in FIG. 17, and no description is provided for them.


In the present embodiment, the n-GaP transparent substrate 1701 and the p-GaP transparent substrate 1708 are affixed to the main body 1750 via a metal.


That is, the LED of the present embodiment has a first metal thin film 2001 formed under the n-Al0.6Ga0.4As current diffusion layer 1702 and a second metal thin film 2002 formed on top of the p-GaP contact layer 1707. It is noted that the first metal thin film 2001 is one example of the first metal material layer, and the second metal thin film 2002 is one example of the second metal material layer.


The manufacturing method of the LED is described below.


First of all, a LED structure wafer 1850 is formed as in the case of the first embodiment. In the case of the present embodiment, it is not necessity to preparatorily form a groove on the LED structure wafer 1850.


Next, by using the vapor deposition method or the sputtering method, a thin film is formed on the epitaxial surface (upper surface of the p-GaP contact layer 1707) of the LED structure wafer 1850 or on the bonding plane (plane to be faced with the LED structure wafer 1850) of the p-GaP transparent substrate 1708.


The thin film may be made of either one of gold, silver, aluminum and titanium; or a compound of gold, silver, aluminum or titanium; or an alloy that contains at least one of gold, silver, aluminum and titanium.


Next, the second metal thin film 2002 is formed by processing the thin film into an arbitrary shape with the photolithography method and wet etching. At this time, the second metal thin film 2002 has an area of not larger than 10% of the element area in forming the element. Thereby, the loss of light at the bonding interface can be suppressed to a minimum.


Next, the p-GaP contact layer 1707 and the p-GaP transparent substrate 1708 are bonded together by means of the bonding jig 1950 (see FIG. 19) as in the case of first embodiment. At this time, the p-GaP contact layer 1707 and the p-GaP transparent substrate 1708 can be bonded together in a heating process carried out for about 30 minutes at a temperature of about 400 to 500° C. under a hydrogen atmosphere.


Next, the substrate and the buffer layer are removed as in the case of first embodiment. Thereafter, the first metal thin film 2001 is formed on the lower surface of the n-Al0.6Ga0.4As current diffusion layer 1702 or on the bonding plane (plane to be faced with the LED structure wafer 1850) of the n-GaP transparent substrate 1702 as in the case of the second metal thin film 2002.


Subsequently, as in the case of the p-GaP transparent substrate 1708 sated above, affixation of the n-GaP transparent substrate 1702 is carried out. Thereafter, electrode forming and chip-making process, which belong to the general manufacturing method of a semiconductor light emitting element, are carried out, so that the LED of the present embodiment is completed.


Third Embodiment


FIG. 21 is a schematic sectional view of a LED according to a third embodiment of the present invention.


The LED of the present embodiment corresponds to a case where one of two transparent substrates is made of an insulator. That is, the LED of the present embodiment has a main body 2150, a glass substrate 2101 placed under the main body 2150, and an n-GaP transparent substrate 2107 placed on top of the main body 2150. It is noted that the glass substrate 2101 is one example of the first transparent substrate, and the n-GaP transparent substrate 2107 is one example of the first transparent substrate.


The main body 2150 is constructed of a p-GaP contact layer 2102, a p-AlInP cladding layer 2103, an AlGaInP active layer 2104, an n-AlInP cladding layer 2105 and an n-GaP contact layer 2106. It is noted that the AlGaInP active layer 2104 is one example of the light emitting layer. Moreover, the p-GaP contact layer 2102 and the p-AlInP cladding layer 2103 constitute one example of the first conductive type semiconductor layer. Then, the n-AlInP cladding layer 2105 and the n-GaP contact layer 2106 constitute one example of the second conductive type semiconductor layer.


The p-AlInP cladding layer 2103 has an exposed part. An electrode 2108 is formed on top of the exposed part. Moreover, an electrode 2109 is formed on top of the n-GaP transparent substrate 2107.


The manufacturing method of the LED is described below.


First of all, as shown in FIG. 22A, the p-GaP contact layer 2102, the p-AlInP cladding layer 2103, the AlGaInP active layer 2104, the n-AlInP cladding layer 2105 and the n-GaP contact layer 2106 are layered in this order by the MOCVD method on a p-GaAs substrate 2111 as one example of the first conductive type semiconductor substrate, so as to form a LED structure wafer 2250.


Next, the n-GaP transparent substrate 2107 is bonded directly to the epitaxial surface (upper surface of the n-GaP contact layer 2105) of the LED structure wafer 2250. That is, the bonding of the n-GaP transparent substrate is carried out without using an adhesive or the like.


The direct bonding of the n-GaP transparent substrate 2107 can be carried out by a method similar to that of the first embodiment.


The surface of the n-GaP transparent substrate has preliminarily been processed for patterning by the photolithography method (using a mask of an oxide of SiO2 etc.), wet etching (by aqua regia, sulfuric acid, oxygenated water mixture solution, etc.) so that a prescribed chip shape can be provided.


Next, the p-GaAs substrate 2111 is removed to provide a state as shown in FIG. 2. Thereafter, the glass substrate 2101 is bonded, with epoxy resin for example, to the removal surface (lower surface of the p-GaP contact layer 2102) of the GaAs substrate, as shown in FIG. 22C.


Next, half dicing is carried out along the pattern of the n-GaP transparent substrate 2107, so that the side surfaces of the n-GaP transparent substrate 2107 are formed to have a slope shape, as shown in FIG. 22D.


The half dicing is carried out by a bevelable blade. Thereby, the side surfaces of the n-GaP transparent substrate 2107 can be formed to have a slope shape.


Next, etching is carried out until the p-GaP contact layer 2102 is exposed with the patterned n-GaP transparent substrate 2107 used as a mask. Etching is conducted by using a mixed liquor of phosphoric acid, sulfuric acid, oxygenated water and water.


Next, as shown in FIG. 21, the electrode 2108 is formed on the exposed portion of the p-AlInP cladding layer 2103, and the electrode 2109 is formed on the n-GaP transparent substrate 2107, and thereafter dicing of the glass substrate 2101 is carried out from the lower surface side, so that the LED of the present embodiment is completed.


As one similar to the LED of the present embodiment, there is a LED wherein the transparent substrate and the semiconductor layer, which is affixed to the substrate, have mutually different conductive types, and wherein no electrical connection is provided between the transparent substrate and the semiconductor layer.


This LED is a LED in which the n-GaP transparent substrate 1701 in the first embodiment is replaced by the p-GaP transparent substrate.


Specifically, the LED is obtained by directly bonding of a p-GaP transparent substrate (carrier density: 5.0×1017 cm−3) for example, as one example of the first transparent substrate, to the n-Al0.6Ga0.4As current diffusion layer 1702 exposed by the removal of the n-GaAs substrate 1801 in the first embodiment.


There is no electrical connection between the p-GaP transparent substrate and the n-Al0.6Ga0.4As current diffusion layer 1702 at the normal LED driving voltage (not higher than 10 V).


The first through third embodiments of the present invention have been described above, the present invention is not limited to the quaternary system LED but applicable to all sorts of semiconductor light emitting elements.


The invention being thus described, it will be obvious that the invention may be varied in many ways. Such variations are not be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims
  • 1. A semiconductor light emitting element comprising: a main body having a first conductive type semiconductor layer, a light emitting layer provided on the first conductive type semiconductor layer and a second conductive type semiconductor layer provided on the light emitting layer;a first transparent substrate placed directly or indirectly under the main body and having transparency with respect to light emitted from the light emitting layer; anda second transparent substrate placed directly or indirectly on top of the main body and having transparency with respect to the light emitted from the light emitting layer.
  • 2. The semiconductor light emitting element as set forth in claim 1, wherein the first transparent substrate is comprised of a first conductive type semiconductor, and the second transparent substrate is comprised of a second conductive type semiconductor.
  • 3. The semiconductor light emitting element as set forth in claim 1, wherein the first transparent substrate is of a second conductive type; orthe second transparent substrate is of a first conductive type; orthe first transparent substrate is of the second conductive type, and the second transparent substrate is of the first conductive type.
  • 4. The semiconductor light emitting element as set forth in claim 2, wherein at least one of the first transparent substrate and the second transparent substrate has a carrier density of not higher than 2.5×1018 cm3.
  • 5. The semiconductor light emitting element as set forth in claim 1, wherein at least one of the first transparent substrate and the second transparent substrate is comprised of an insulator.
  • 6. The semiconductor light emitting element as set forth in claim 1, wherein at least one of the first transparent substrate and the second transparent substrate has a slope surface inclined to the upper surface of the light emitting layer.
  • 7. The semiconductor light emitting element as set forth in claim 1, wherein a light emitting region in the main body is located near a center of the main body as viewed cross-sectionally.
  • 8. The semiconductor light emitting element as set forth in claim 1, further comprising: a current constriction structure for locating a light emitting region near a center of the main body as viewed cross-sectionally.
  • 9. The semiconductor light emitting element as set forth in claim 1, wherein the light emitting layer has a structure stacked with semiconductor crystals comprised of two or more elements of gallium, aluminum, indium, phosphorus, arsenic, zinc, tellurium, sulfur, nitrogen, silicon, carbon and oxygen.
  • 10. A semiconductor light emitting element manufacturing method comprising the steps of: successively layering a first conductive type semiconductor layer, a light emitting layer and a second conductive type semiconductor layer on a first conductive type semiconductor substrate;bonding a second transparent substrate having transparency with respect to light emitted from the light emitting layer to an upper surface of the second conductive type semiconductor layer; andbonding a first transparent substrate having transparency with respect to light emitted from the light emitting layer to a lower surface of the first conductive type semiconductor layer by removing the first conductive type semiconductor substrate after the step of bonding the second transparent substrate.
  • 11. The semiconductor light emitting element manufacturing method as set forth in claim 10, wherein the second transparent substrate is bonded directly to the upper surface of the second conductive type semiconductor layer by processing under pressure and heating in the step of bonding the second transparent substrate.
  • 12. The semiconductor light emitting element manufacturing method as set forth in claim 10, wherein the first transparent substrate is bonded directly to the lower surface of the first conductive type semiconductor layer by processing under pressure and heating in the step of bonding the first transparent substrate.
  • 13. The semiconductor light emitting element manufacturing method as set forth in claim 10, wherein the second transparent substrate is bonded to the upper surface of the second conductive type semiconductor layer via a second transparent material layer that has transparency with respect to light emitted from the light emitting layer in the step of bonding the second transparent substrate.
  • 14. The semiconductor light emitting element manufacturing method as set forth in claim 10, wherein the first transparent substrate is bonded to the lower surface of the first conductive type semiconductor layer via a first transparent material layer that has transparency with respect to light emitted from the light emitting layer in the step of bonding the first transparent substrate.
  • 15. The semiconductor light emitting element manufacturing method as set forth in claim 10, wherein the second transparent substrate is bonded to the upper surface of the second conductive type semiconductor layer via a second metal material layer of an arbitrary shape in the step of bonding the second transparent substrate.
  • 16. The semiconductor light emitting element manufacturing method as set forth in claim 10, wherein the first transparent substrate is bonded to the lower surface of the first conductive type semiconductor layer via a first metal material layer of an arbitrary shape in the step of bonding the first transparent substrate.
  • 17. The semiconductor light emitting element manufacturing method as set forth in claim 11, wherein the step of bonding the first transparent substrate and the step of bonding the second transparent substrate are different from each other in bonding methods.
  • 18. A compound semiconductor light emitting diode manufactured by using the semiconductor light emitting element manufacturing method set forth in claim 10, wherein the light emitting layer has a structure stacked with semiconductor crystals comprised of two or more elements of gallium, aluminum, indium, phosphorus, arsenic, zinc, tellurium, sulfur, nitrogen, silicon, carbon and oxygen.
  • 19. The semiconductor light emitting element as set forth in claim 3, wherein at least one of the first transparent substrate and the second transparent substrate has a carrier density of not higher than 2.5×1018 cm−3.
  • 20. The semiconductor light emitting element manufacturing method as set forth in claim 11, wherein the first transparent substrate is bonded directly to the lower surface of the first conductive type semiconductor layer by processing under pressure and heating in the step of bonding the first transparent substrate.
  • 21. The semiconductor light emitting element manufacturing method as set forth in claim 13, wherein the step of bonding the first transparent substrate and the step of bonding the second transparent substrate are different from each other in bonding methods.
Priority Claims (1)
Number Date Country Kind
2006-169700 Jun 2006 JP national
CROSS-REFERENCE TO RELATED APPLICATION

This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2006-169700 filed in Japan on 20 Jun. 2006, the entire contents of which are incorporated herein by reference.