Patent Publication Number: US-2002003637-A1

Title: Method for alignment of a hologram with an associated substrate

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates generally to a hologram alignment mark and a method for fabricating the same, and more particularly to a hologram alignment mark located at a given position on a hologram surface and fabricated by a hologram replication process and a method for fabricating the same.  
       [0002] A hologram product should align precisely with a substrate, etc., on which it is to be mounted. For instance, a hologram color filter for liquid crystal display devices proposed by the applicant in Japanese Patent Application No. 5-12170 should be in precise alignment with one of liquid crystal display device substrates.  
       [0003] In this regard, the applicant has filed Japanese Patent Application No. 7-223081 to come up with an alignment mark and method for incorporating a hologram color filter in a liquid crystal display device.  
       [0004] Referring here to the hologram color filter, it comprises an eccentric Fresnel zone plate type of micro-hologram array. Another hologram color filter has also been proposed in the art, which comprises a hologram or diffraction grating made up of parallel and uniform interference fringes and a condensing lens array located on an entrance or exit side thereof. A typical hologram color filter comprising a decentering Fresnel zone plate form of micro-hologram array is here briefly explained.  
       [0005] A liquid crystal display device using the above-mentioned hologram color filter is now explained with reference to a sectional view attached hereto as FIG. 11. As shown, a hologram color filter-forming hologram array  5  is spaced away from the side of a liquid crystal display device  6  regularly divided into liquid crystal cells (pixels)  6 ′, on which backlight  3  is to be incident. On the back side of the liquid crystal display device  6 , a black matrix  4  is located between adjacent liquid crystal cells  6 ′. Although not illustrated, polarizing plates are positioned on both sides of the liquid crystal display device  6 . In this arrangement, it is acceptable to additionally locate between adjacent black matrices  4  an absorption type color filter for transmitting light of colors corresponding to R, G, and B pixels, as is the case with a conventional color liquid crystal display device.  
       [0006] The hologram array  5  comprises an array of micro-holograms  5 ′ arranged at the same pitch as the repetitive period of R, G, and B color pixels, i.e., corresponding to each set of three liquid crystal cells  6 ′ in the liquid crystal display device  6 , which cells are adjacent to one another on the paper. The micro-hologram  5 ′ are arranged one by one in alignment with each set of three liquid crystal cells  6 ′ in the liquid crystal display device  6 , which cells are adjacent to one another on the paper. In each micro-hologram  5 ′, interference fringes are formed into a decentering Fresnel zone plate shape, so that a green component of the backlight  3  incident at an angle θ with respect to the normal to the hologram array  5  can be focused on the central liquid crystal cell G of the three R, G and B color pixels corresponding to the micro-hologram  5 ′ (an eccentric hologram lens). The micro-hologram  5 ′ is made up of a transmission hologram of the relief, phase or amplitude type with diffraction efficiency having no or little dependence on wavelength. By the “type with diffraction efficiency having no or little dependence on wavelength” used herein is intended a hologram type capable of diffracting every wavelength with one single diffraction grating, not a hologram type designed to diffract a specific wavelength alone and not to diffract other wavelengths substantially, as is the case with a Lippmann type hologram. A diffraction grating with diffraction efficiency less depending on wavelength diffracts light at an angle of diffraction varying depending on wavelength.  
       [0007] Upon incidence of the white backlight  3  striking from the side of the hologram array  5 , which faces away from the liquid crystal display device  6 , at the angle θ with respect to the normal thereto, the angle of diffraction through the micro-hologram  5 ′ varies depending on wavelength, and the focusing position with respect to each wavelength is dispersed in a direction substantially parallel with the surface of the hologram array  5 . If the hologram array  5  is designed and arranged in such a manner that red, green, and red wavelength components are diffracted and focused on the positions of a red emitting liquid crystal cell R, a green emitting liquid crystal cell G, and a blue emitting liquid crystal cell B, respectively, the respective color components can then transmit through the respective liquid crystal cells  6 ′ without substantial attenuation, presenting color displays depending on the states of the liquid crystal cells  6 ′ at the corresponding positions.  
       [0008] By using the hologram array  5  as a color filter in this manner, it is possible to make the most efficient use of backlight for a conventional color filter because the respective wavelength components of the backlight can be incident on the respective liquid crystal cells  6 ′ with efficiency and without absorption.  
       [0009] The color filter  5  comprising such a hologram array as mentioned above may be fabricated by a replication process making use of two-beam interference of +first-order multi-point converging light diffracted from a micro-hologram lens array comprising, e.g., a computer-generated hologram, and zero-order transmitted light. This replication process is now briefly explained with reference to a sectional view attached hereto as FIG. 10. Hologram interference fringes for the micro-hologram  5 ′ (FIG. 11) are computed by a computer. The fringes are written with an electron beam on a glass substrate  1  on which an electron beam resist, for instance, is coated, and then developed to fabricate an array  7 ′ of a relief type of computer-generated hologram (CGH)  5 ′. As shown in FIG. 10, a hologram photosensitive material  17  comprising a glass substrate  12 , a photopolymer or other photosensitive layer  13  provided thereon and a cover film  14  laminated on the layer  13  is placed on a hologram pattern  2  on the thus obtained CGH array  7 ′ serving as a hologram plate in a contact or slightly spaced relation thereto. Then, laser light  9  is incident on the CGH array  7 ′ at an angle θ corresponding to that of the backlight  3  in FIG. 11 to replicate the CGH array  7 ′ by interference of converging diffracted light  10 ′formed by each CGH  5 ″ in the CGH array  7 ′ and straightforward transmitted light  1  in the photosensitive layer  13  in the hologram photosensitive material  18 . It is acceptable to carry out a similar replication process using the thus replicated hologram as a hologram plate, thereby forming the hologram array  5 . It is noted that the angle of incidence of the laser light  9  for replication is not necessarily almost equal to the angle θ of incidence of the backlight  3 , and the wavelength of the laser light is not necessarily almost equal to that of the backlight  3 .  
       [0010] To built a hologram product such as the above hologram color filter  5  in a device to be used therewith, etc., it is required that the hologram product be in precise alignment with a substrate on which it is to be mounted, e.g., a substrate with black matrices  4  provided thereon in the case of the hologram color filter  5 .  
       [0011] So far, an alignment mark has been fabricated by providing a patterned metal film on a glass substrate by an evaporation process, etc., and then coating a hologram photosensitive material for hologram replication on the glass substrate.  
       [0012] When a hologram is replicated from a hologram plate on the basis of the alignment mark on the substrate, however, a position error is introduced between the hologram plate and the substrate coated with the hologram photosensitive material, resulting in a drop of the position precision of the alignment mark on the end product. This position precision becomes worse as hologram replication is repeated using the replicated hologram as a hologram plate, resulting in some considerable misalignment of the alignment mark on the end product.  
       SUMMARY OF THE INVENTION  
       [0013] In view of such prior art problems as mentioned above, an object of the present invention is to provide a hologram alignment mark which can be fabricated by a hologram replication process while it is in good alignment with an alignment mark on the hologram plate to be replicated, and a method for fabricating the same.  
       [0014] According to the present invention, this object is achieved by the provision of a hologram alignment mark which is an alignment mark provided on the same substrate as a hologram substrate, and comprises a reflection hologram having a predetermined outside shape.  
       [0015] Preferably in this case, the reflection hologram comprises a hologram layer wherein interference fringes are arranged parallel with one another on a hologram layer surface.  
       [0016] For instance, the hologram provided on the same substrate as provided with the alignment mark comprising the reflection hologram may be a transmission hologram.  
       [0017] The present invention also provides a method for fabricating a hologram alignment mark by providing a reflecting member having a predetermined pattern on the same substrate as a substrate for the hologram plate to be replicated, and replicating a hologram from said hologram plate by a hologram replication process, characterized in that by incidence of illumination light on said reflecting member, a reflection hologram having an outside shape corresponding to said pattern on said reflecting member is recorded at a position of a surface of said hologram replicated from said hologram plate, said position corresponding to said reflecting member.  
       [0018] For instance, the hologram plate may comprise a phase hologram which is replicated by a transmission hologram replication process. The reflecting member may be provided at a position of a part of the phase hologram surface.  
       [0019] Moreover, the present invention includes a hologram alignment mark reading method wherein a laser is used as an illumination light source for the reflection hologram or a white light source is used in combination with a filter having a wavelength range substantially identical with a diffraction wavelength of the reflection hologram.  
       [0020] The hologram alignment mark of the present invention is an alignment mark which is provided on the same substrate as a hologram substrate and comprises a reflection hologram having a predetermined outside shape. Even when the replication process is repeated to fabricate holograms, it is thus unnecessary to provide another alignment mark on the glass substrate of the end product, because the alignment mark of the invention is unsusceptible to any displacement and is of high precision.  
       [0021] Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.  
       [0022] The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023]FIG. 1 is a perspective view of one embodiment of the hologram product provided thereon with the hologram alignment mark of the invention.  
     [0024]FIG. 2( a ) illustrates one shape of an embodiment of the hologram alignment mark of the invention, and FIG. 2( b ) shows one section of that embodiment.  
     [0025]FIG. 3 is a sectional view of one arrangement for replicating a hologram from a hologram plate by a hologram replication process.  
     [0026]FIG. 4 illustrates one embodiment of the outside shape of an alignment mark chromium pattern provided on the hologram plate.  
     [0027]FIG. 5 illustrates one arrangement for carrying out alignment using the hologram alignment mark of the invention.  
     [0028]FIG. 6 illustrates one example of the alignment display monitor screen.  
     [0029]FIG. 7 is an illustration of how the main hologram and hologram alignment mark are replicated using the hologram replicated in the FIG. 3 arrangement as a hologram plate.  
     [0030] FIGS.  8 ( a ) and  8 ( b ) are views of modifications of the FIGS. 3 and 7 replication processes.  
     [0031] FIGS.  9 ( a ),  9 ( b ) and  9 ( c ) are views of an embodiment wherein replication efficiency is improved by replicating a plurality of holograms using a large hologram plate.  
     [0032]FIG. 10 is a sectional view illustrative of a process for replicating a hologram from a hologram plate.  
     [0033]FIG. 11 is a sectional view of a liquid crystal display device using a hologram color filter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0034] The hologram alignment mark and hologram alignment mark fabrication method according to the invention will now be explained with reference to embodiments wherein the hologram alignment mark is used in combination with a hologram color filter.  
     [0035]FIG. 1 is a perspective view of a hologram color filter representative of a hologram product. The hologram  20  comprises a hologram layer  13  provided on a glass substrate  12  and formed of a photopolymer or the like. A main hologram  21  comprising a hologram array  5  is recorded on a substantially central region of the hologram layer  13 , and hologram alignment marks  22  are provided on the centers of two sides of the main hologram  21  in predetermined position relations thereto. Both the main hologram  21  and the hologram alignment marks  22  comprise phase type volume holograms. More specifically, the main hologram  21  comprises a transmission hologram which is usually transparent and invisible. Each hologram alignment mark  22  is a reflection hologram which, as shown in the FIG. 2( b ) sectional view, comprises a so-called hologram mirror made up of interference fringes  24  arranged parallel with one another on a hologram surface of the hologram layer  13 . In the invention, the outside shape (or contour) of the reflection hologram  22  is utilized as an alignment mark. The hologram alignment mark  22  has a crosswise outside shape as typically shown in FIG. 2( a ).  
     [0036] For a better understanding of the hologram alignment mark  22  according to the invention, its fabrication method is first explained. FIG. 3 is a sectional view of an arrangement for recording the main hologram  21  and hologram alignment marks  22  from a hologram plate  25  to be replicated on the same hologram photosensitive material  18  by a hologram replication process. As already explained with reference to FIG. 10, a glass substrate  26  of the hologram plate  25  is provided on a substantially central region of its surface with a main hologram pattern  27  representing hologram interference fringes for the main hologram  21 . At the same time, alignment mark chromium patterns  28  are provided in predetermined position relations to the hologram pattern  27 . When the hologram alignment mark  22  has a crosswise outside shape as shown in FIG. 2( a ), the outside shape of the alignment mark chromium pattern  28  should be congruent with the outside shape of the hologram alignment mark  22 , as shown in FIG. 4. In this case, at least the alignment mark chromium pattern  28  should be of reflection capability. Then, a hologram photosensitive layer  18  comprising a glass substrate  12 , a photosensitive layer  13  formed of a photopolymer or the like and provided on the glass substrate  12  and a cover film  14  laminated on the photosensitive layer  13  is brought into close contact on the cover film  14  with the alignment mark chromium pattern  28 . It is noted that the surface of the glass substrate  12  that faces away from the photosensitive layer  13  in the hologram photosensitive material  18  is provided with an absorption layer  19  which absorbs laser light  9  for the replication of the main hologram and transmits, or does not absorb, laser light  29  for the replication of the hologram alignment marks.  
     [0037] In such an arrangement, the laser light  9  is incident from the side of the hologram plate  25  on the region of the main hologram pattern  27 , so that diffracted light  10 ′ and straightforward transmitted light  11  can interfere with each other in the photosensitive layer  13  in the hologram photosensitive material  18  to replicate the main hologram  21 . At the same time as, prior to, or after this, another laser light  29  is incident from the side of the hologram photosensitive material  18  vertically on the region of the alignment mark chromium pattern  28 , so that the incident light and light  30  reflected from the alignment mark chromium pattern  28  can interfere with each other in the photosensitive layer  13  in the hologram photosensitive material  18  to fabricate (or replicate) the hologram alignment mark  22 . This hologram alignment mark  22  is made up of interference fringes formed by light beams  19  and  20  propagating vertically with respect to the photosensitive layer  13  and in opposite directions to each other. That is, the hologram alignment mark  22  is made up of interference fringes  24  arranged parallel with one another on the hologram surface of the hologram layer  13 , as shown in FIG. 2( b ). The outside shape of the region with the interference fringes  24  recorded thereon is such that the alignment mark chromium pattern  28  is proximate to the photosensitive layer  13  in the hologram photosensitive material  18 , and so is the same as that of the alignment mark chromium pattern  28 . To fabricate the hologram alignment mark  22  having the same outside shape as that of the alignment mark chromium pattern  28 , it is desired that the cover film  14  in the hologram photosensitive material  18  be as thin as possible, and formed of a material free from double refraction capability.  
     [0038] The absorption layer  19  provided on the surface of the substrate  12  that faces away from the photosensitive layer  13  in the hologram photosensitive material  18  is capable of absorbing the laser light  9  for the replication of the main hologram and incapable of absorbing the laser light  29  for the replication of the hologram alignment mark. When the main hologram  21  is replicated, it is thus possible to prevent multiple reflection of laser light in the glass substrate  12 , which may otherwise allow unnecessary interference fringes to be recorded thereon. However, this absorption layer  19  forms no impediment to the replication of the hologram alignment mark  22  because of being capable of transmitting the laser light  29  having a wavelength different from that of the laser light  9 . It is noted that when laser light having the same wavelength range is used for the laser light  9  and laser light  29 , it is unnecessary to provide the absorption layer  19  on a region corresponding to the alignment mark chromium pattern  28 .  
     [0039] To bring the hologram  20  which is provided on its periphery with the hologram alignment marks  22  as mentioned above in precise alignment with an application substrate  31  of a liquid crystal display device  6  (FIG. 11) provided on its back side with black matrices  4 , etc., the hologram  20  and application substrate  31  are brought close to each other, as shown in FIG. 5, so that the hologram alignment marks  22  on the hologram  20  can oppose to alignment marks  32  on the application substrate  31 . Each alignment mark  32  may be formed of a pattern which enables an misalignment, if any, from the associated hologram alignment mark  22  to be easily detected. When the hologram alignment mark  22  has a crosswise shape as shown in FIG. 6 (an image  22 ″ of the mark  22  is shown in FIG. 6), the alignment mark  32 , too, should have the same crosswise shape (an image  32 ″ of the mark  32  is shown in FIG. 6). For instance, when the application substrate  31  is the substrate of the liquid crystal display device  6  provided with black matrices  4 , the alignment mark  32  is formed of a metal or other mark having opaque contrast.  
     [0040] To bring the hologram  20  in precise alignment with the application substrate  31 , light from a white light source or a monochromatic light source  33 , for instance, is converted by an optical system  34  into parallel light, which is then incident vertically on the side of the hologram  20  through a half-silvered mirror  35  and an objective  36 . For the monochromatic light source  33 , a laser may be used or a white lamp may be used in combination with a filter having the same wavelength range as the diffraction wavelength of the hologram alignment mark  22 . Then, an superposed and magnified image of the hologram alignment mark  22  and alignment mark  32  is phototaken by a CCD  37  thorough the objective  36  to display the phototaken magnified image on a monitor screen  38 . For the objective  36  it is desired to use a telecentric objective.  
     [0041] One example of the monitor screen is shown in FIG. 6. Light of a given wavelength determined depending on the spacing between the interference fringes  24  (FIG. 2( b )) is reflected from the hologram alignment mark  22  on the hologram  20  to display on the monitor screen the monochromatic and crosswise image  22 ″ of the hologram alignment mark  22 , which represents the outside shape of the hologram alignment mark  22  and has high luminance. On the other hand, the same crosswise image  32 ″ is displayed as a background of the image  22 ″ from the alignment mark  32  on the application substrate  31 . By controlling the positions of the hologram  20  and application substrate  31  to bring the crosswise shapes of the images  32 ″ and  22 ′ into position alignment with each other, it is thus possible to bring both into precise alignment with each other. When a monochromatic light source is used as the light source  33 , it is to be noted that the light emission wavelength should be in agreement with the Bragg wavelength determined depending on the spacing between the interference fringes  24  forming the hologram alignment mark  22 . Upon development of the photosensitive layer  13 , the spacing between the interference fringes  24  becomes slightly narrow. It is thus required that the wavelength of the monochromatic light source  33  be given by λ(1−Δ) where λ is the wavelength of the laser light  29  upon recording, and Δ is the rate of shrinkage of the photosensitive layer  13 . When a white light source is used as the light source  33 , it is to be noted that there may be a slight lowering of the contrast of the images  22 ″ and  32 ″ of the alignment marks.  
     [0042] While the invention has been explained with reference to the alignment mark chromium pattern  28  having a crosswise outside shape, i.e., the hologram alignment mark  22  having a crosswise outside shape, it is understood that the hologram alignment mark  22  may have other various outside shapes, and three or more hologram alignment marks may be used. With these modifications, the alignment mark  32  on the application substrate  32 , too, may be selected or modified as to shape and position.  
     [0043] While a transmission hologram is used as the main hologram  21  of the hologram  20 , it is understood that the invention may also be applied to a reflection hologram.  
     [0044] Next, an account will be given of how to fabricate the hologram alignment mark  22  when a hologram product is fabricated by carrying out a hologram replication process using the replicated hologram as a hologram plate.  
     [0045]FIG. 7 is an illustration of how the main hologram  21  and hologram alignment marks  22  are replicated on another hologram photosensitive material  18  using as a hologram plate the hologram  20  replicated from the hologram plate  25  according to the arrangement shown in FIG. 3. In the FIG. 3 embodiment, the reflecting member provided on the plate  25  for the replication of the hologram alignment marks  22  is formed of the chromium pattern  28 . In the FIG. 7 embodiment, however, the hologram alignment marks  22  are each a hologram of the reflection type recorded by light reflected from each chromium pattern  28 . Upon development of the photosensitive layer  13  after the first replication in the FIG. 3 embodiment, the spacing between the interference fringes  24  forming each hologram alignment mark  22  becomes usually somewhat narrow, as already mentioned. It is thus required that the wavelength of the vertically incident laser light  29  for the replication of each hologram alignment mark  22  at the second replication time in the FIG. 7 embodiment be given by (1−Δ) where λ is the first wavelength, and Δ is the rate of shrinkage of the photosensitive layer  13 .  
     [0046]FIG. 8 is a modification of the FIG. 7 embodiment. In this modification, laser light  9  is first incident from the hologram plate  25  on a region of the main hologram pattern  27  to replicate the main hologram  21  alone, as shown in FIG. 8( a ). However, reflecting patterns  28 ′ having the same shape as that of each alignment mark chromium pattern  28  on the hologram plate  25  are provided on the glass substrate  12  in the hologram photosensitive material  18 . The main hologram  21  is then replicated after the alignment of both using the alignment mark chromium patterns  28  on the hologram plate  25  and the reflecting patterns  28 ′ on the hologram photosensitive material  18 .  
     [0047] As shown in FIG. 80( b ), the main hologram  21  is then replicated as in the FIG. 7 embodiment, using the thus replicated hologram plate  20 ′. At the same time, the hologram alignment marks  22  are fabricated (or replicated) as in the FIG. 3 embodiment, using the reflecting patterns  28 ′.  
     [0048] A difference between the FIG. 3+FIG. 4 method and the FIG. 8 method is that the amount of a blur of the hologram alignment mark  22 ′ in the end product (FIG. 9) is larger in the former than in the latter because the once fabricated hologram alignment mark  22  is again replicated.  
     [0049] In another modification, a reflecting pattern  28 ′ having a shape corresponding to the hologram alignment mark  22  on the hologram plate  20  is provided on the glass substrate  12  of another hologram photosensitive material  19  in the FIG. 7 replication embodiment, as shown in FIG. 8( a ). After alignment of both using the hologram alignment marks  22  on the hologram plate  20  and the reflecting patterns  28 ′ on the hologram photosensitive material  18 , the laser light  9  is incident on the region of the main hologram  21  to replicate the main hologram  21  alone.  
     [0050] Illustrated in FIG. 9 is an embodiment wherein the efficiency of using the hologram replicated from the first hologram plate as another hologram plate is improved by providing a plurality of holograms on a hologram photosensitive material. Using such a hologram plate  25  (see FIG. 3) as shown in FIG. 9( a ), a plurality (six in this embodiment) of holograms  20  are arranged side by side on a large hologram photosensitive material  18  as shown in FIG. 9( b ) for replication purposes. This replication may be carried out either by the FIG. 3 method or by the FIG. 8( a ) method. Using the photosensitive material  18  with a plurality of holograms provided thereon as a hologram plate, a plurality of hologram  20  are then replicated at a time on another large photosensitive material  18 , as shown in FIG. 9( c ). In FIG. 9,  20 ′ represents a hologram fabricated at the second replication step,  21 ′ a main hologram, and  22 ′ a hologram alignment mark.  
     [0051] While the hologram alignment mark of the invention and its fabrication method have been described with reference to some preferred embodiments, it is understood that the invention is not limited thereto, and so various modification may be made thereto.  
     [0052] As can be understood from the foregoing, the present invention is an alignment mark which is provided on the same substrate as a hologram substrate and comprises a reflection hologram having a predetermined outside shape. Even when the replication process is repeated to fabricate holograms, it is thus unnecessary to provide another alignment mark on the glass substrate of the end product, because the alignment mark of the invention is unsusceptible to any displacement and is of high precision. The present invention also provides a method for fabricating such an alignment mark.