JP2006139050A - Multilayer substrate, and reflective liquid crystal display element using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer substrate which is used in a reflective liquid crystal display device and which has a layer extremely efficiently reflecting visible rays, and also to provide a reflective liquid crystal display device using the multilayer substrate, comparatively easily and at a low cost. <P>SOLUTION: The multilayer substrate has at least a supporting substrate, a photonic crystal formed on a surface of the supporting substrate, and an element forming layer formed on a surface of the photonic crystal and having a semiconductor element fabricated thereon, wherein the photonic crystal is a periodic structural body comprising two or more kinds of layers which have refractive indexes with respect to visible rays different from one another and which are periodically aligned, and further the periodic structural body has layer thickness of one period thereof adjusted so as to be a one-dimensional photonic crystal with respect to the visible rays. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is used to form a reflective liquid crystal display element used in a reflective liquid crystal projector, a high resolution projection TV, and the like, and uses a multilayer substrate having a layer that reflects visible light extremely efficiently, and the multilayer substrate. The present invention relates to a reflection type liquid crystal display element.

  As a liquid crystal projector using a liquid crystal display element, a transmission type liquid crystal projector in which a liquid crystal driving element is formed on a polysilicon layer deposited on a transparent substrate is widely marketed, but a high-temperature polysilicon device is used for liquid crystal driving. Therefore, the driving speed of the peripheral circuit is small, and there is a delay specific to liquid crystal.

  In recent years, a reflective liquid crystal projector using a structure called LCOS (Liquid Crystal on Silicon), which is expected to be inexpensive for mass production, has been researched and developed in place of the conventional transmissive liquid crystal projector. ing.

A cross-sectional structure of a reflective liquid crystal display element 200 using a general LCOS structure is shown in FIG. Basically, a liquid crystal is placed on the silicon device 202, and an electric field is formed between a reflective electrode 204 (which is an electrode and a reflective layer) and an ITO (Indium Tin Oxide) electrode 207 on the uppermost layer. To control the On-Off of the liquid crystal. The reflective electrode 204 is connected to the underlying silicon substrate 201, and can receive an electrical signal from this to control the electric field.
Japanese Patent Application Laid-Open No. 9-203884 JP-A-11-142869

This LCOS has advantages such as an aperture ratio of 90% or higher, high resolution, high brightness, and high contrast. However, the complex light path, that is, the use of a prism or an external reflector does not greatly exceed the transmission type characteristics. Further problematic is the reflectivity of the metal reflective layer with respect to visible light.

In FIG. 2, incident visible light is reflected by the reflective electrode located under the liquid crystal, and an image is displayed by the reflected light. Therefore, in order to obtain a high-luminance image, the reflectivity is close to 100%. desired. However, the reflectances of aluminum and silver that are usually used as reflective electrodes are about 90% and 96%, respectively, and are not sufficient. In addition to the reflection by metal, a dielectric film having a multilayer structure may provide a reflectivity exceeding 96%. Stacking more than one layer is necessary, resulting in an increase in cost, and electric charges are generated by light irradiation, which hinders driving of the liquid crystal.

The present invention has been made to solve the above-described problems, and is used in a reflective liquid crystal display device, and includes a multilayer substrate having a layer that reflects visible light extremely efficiently, and a reflective liquid crystal using the multilayer substrate. It is an object to provide a display device relatively easily and at low cost.

In order to achieve the above object, a multilayer substrate of the present invention includes at least a support substrate, a photonic crystal formed on the surface of the support substrate, and an element on which a semiconductor element is formed by forming the surface of the photonic crystal. A multi-layer substrate, wherein the photonic crystal is a periodic structure in which two or more layers having different refractive indices with respect to visible light are periodically arranged, and the periodic structure Is characterized in that the layer thickness of one period is adjusted so as to form a one-dimensional photonic crystal with respect to the visible light.

As described above, the one-dimensional photonic crystal as the visible light reflecting layer is formed under the element forming layer for forming the semiconductor element (sometimes referred to as a device), so that the visible light transmitted through the element forming layer is transmitted. Can be efficiently reflected.

In this case, in each layer constituting one cycle of the periodic structure, it is preferable that a difference in refractive index between the layer having the maximum refractive index with respect to the visible light and the layer having the minimum refractive index is 1.0 or more. The wavelength width of the reflected visible light increases as the refractive index difference increases within one period of the periodic structure. Therefore, the reflection in a wider wavelength range can be made more reliable by increasing the refractive index difference. Therefore, the difference in refractive index is preferably 1.2 or more, more preferably 1.5 or more.

As a specific layer for forming the photonic crystal, it is preferable that the layer having the maximum refractive index with respect to visible light is the Si layer and the layer having the minimum refractive index is the SiO ₂ layer. These can be easily formed by the CVD method, and the refractive index difference is an extremely large value of about 2.0.

In consideration of the manufacturing cost of the photonic crystal, it is preferable that the periodic structure is composed of two types of layers and the number of periods is 5 or less. Even with such a small number of periods, the layer thickness of one period is adjusted so that it becomes a one-dimensional photonic crystal with respect to visible light, so that an extremely high reflectance can be obtained. As a result, a multilayer substrate having a reflectance of 98% or more can be obtained in a wavelength band of at least 100 nm in the entire wavelength region of visible light.

  Peripheral circuits such as devices necessary for liquid crystal driving are formed on a part of the element formation layer of such a multilayer substrate, and visible light incident on the surface of a part of the element formation layer on which the peripheral circuit is formed. A reflective film for reflection is formed. Even if such a reflective film is not formed in the element formation layer region where the remaining peripheral circuits are not formed, the incident visible light can be sufficiently reflected by the photonic crystal formed on the base.

Furthermore, it is possible to form a reflective liquid crystal display element by forming a structure in which at least a liquid crystal layer and a transparent substrate are laminated on the element forming layer on which the peripheral circuit and the reflective film are formed. At this time, the element formation layer formed on the surface of the photonic crystal as a layer for forming a device necessary for driving the liquid crystal is preferably single crystal silicon. By forming peripheral circuits related to liquid crystal drive with single crystal silicon in this way, the speed of the drive element is increased compared to the case where circuit elements are formed with high-temperature polycrystalline silicon, thereby preventing delays peculiar to liquid crystals. Can do.

According to the present invention, a multilayer substrate having a layer that reflects visible light very efficiently, and a reflective liquid crystal display device using the multilayer substrate, which is used for forming a reflective liquid crystal display element, are relatively simple and simple. It can be obtained at low cost. In addition, since the liquid crystal driving circuit can be formed of a single crystal silicon layer, the speed of the driving element can be improved.
Furthermore, since the infrared light incident from the surface of the reflective liquid crystal display element passes through the photonic crystal and the support substrate, an increase in the liquid crystal temperature can be suppressed.

Hereinafter, although embodiment of this invention is described, this invention is not limited to these.
In FIG. 1, a semiconductor element 104 is formed in a partial region of an element formation layer 103 which is the uppermost layer of a multilayer substrate according to the present invention, and a reflective film 109 is formed on the outermost surface of the semiconductor element 104. FIG. 2 is a schematic cross-sectional view of a reflective liquid crystal display element 100 having a structure in which a liquid crystal layer 105 and a transparent substrate 108 are laminated on an ITO electrode 107 and an alignment film 106 thereon.

The multilayer substrate and reflective liquid crystal display device of the present invention having such a structure can be manufactured by the following steps.
First, a silicon wafer to be the support substrate 101 is prepared. As a support substrate, a substrate made of glass, quartz, sapphire, etc. can be used in addition to a silicon single crystal wafer, but since a device such as C-MOS or SRAM is formed in a later process, a mature LSI process is applied as it is. A silicon wafer is advantageous in terms of the cost of the substrate material.

Next, a stacked body is formed on the surface of the support substrate 101 by stacking a plurality of periodic structures 102 in which two or more layers having different refractive indexes are periodically arranged. The periodic structure 102 is adjusted to have a layer thickness of one period so as to be a one-dimensional photonic crystal with respect to visible light. Specifically, as shown in FIG. 1, a pair of the high refractive index layer 10 and the low refractive index layer 11 is defined as one cycle, and the layer thickness of the one cycle is lower than that of the high refractive index layer 10 for each visible light. The refractive index layer 11 is adjusted so as to correspond to an integral multiple of the half wavelength (λa / 2) of the medium average wavelength λa obtained by averaging the wavelengths in the medium.

In the periodic structure configured as described above, as shown in the schematic diagram of FIG. 4, the refractive index periodically changes in the stacking direction. The length of one period in the periodic change of the refractive index is a half wavelength of propagating light that is to propagate in the stacking direction in the periodic structure, that is, an integer of the half wavelength (λa / 2) of the average wavelength in the medium. When it corresponds to double, such propagating light cannot propagate through the periodic structure and is reflected in a form close to complete reflection. Such a phenomenon of reflecting light in a specific wavelength region is generally called a photonic band gap because it has the same concept as a band gap explained by a dispersion relationship of electrons in a solid crystal of a semiconductor or the like. In particular, a material having a photonic band gap only for light propagating in the stacking direction, such as a periodic structure, is called a one-dimensional photonic crystal.

FIG. 1 shows an example in which a three-period periodic structure having a low refractive index layer 11 and a high refractive index layer 10 as one period is formed as a one-dimensional photonic crystal on a support substrate. When the low refractive index layer 11 is an Si layer and the high refractive index layer 10 is an SiO ₂ layer, this periodic structure has a thermal oxidation process for forming the SiO ₂ layer and a CVD (Chemical Vapor Deposition) process for forming the Si layer. It can be formed by repeating. It is possible to form not only the Si layer but also the SiO ₂ layer by CVD. In addition to the CVD method, it can also be formed by using a well-known thin film growth method such as a metal organic vapor phase epitaxy (MOVPE) method, a molecular beam epitaxy (MBE) method, or a sputtering method including high-frequency sputtering or magnetron sputtering. . Furthermore, a method called “Smart Cut (registered trademark)” (refer to Japanese Patent Laid-Open No. 5-211128) is repeated, in which hydrogen ions are implanted into one wafer and then bonded to the other wafer, followed by heat treatment to peel off the thin film. By doing so, a photonic crystal with extremely high film thickness uniformity can be obtained.

  One feature of the multilayer substrate of the present invention is that it has an element formation layer that is formed on the surface of a photonic crystal to produce a semiconductor element. As the element formation layer, a Si layer may be formed again after the photonic crystal is formed. However, when the Si layer is formed on the uppermost layer of the photonic crystal, the Si layer is used as the element formation layer. be able to. As the element formation layer, a polycrystalline silicon layer formed by a CVD method or the like can be used. However, as described above, in order to improve the operation speed of the liquid crystal peripheral circuit, it is preferable to use a single crystal silicon layer. is there. A method for forming a single crystal silicon layer on the uppermost layer of the photonic crystal (three cycles) will be described with reference to FIG.

In FIG. 3, a support substrate 51 made of a mirror-polished silicon single crystal wafer and a material wafer 52 for forming a single crystal silicon layer on the uppermost layer are prepared. Then, on the surface of the support substrate 51, SiO ₂ layers 54 and Si layers 53 for forming a photonic crystal are alternately formed in two periods. As described above, this periodic structure can be formed, for example, by repeating the thermal oxidation process for forming the SiO ₂ layer and the CVD process for forming the Si layer. The process of forming the SiO ₂ layer can also be a CVD process using TEOS (tetraethoxysilane) or the like.

On the other hand, on the surface of the material wafer 52, the periodic structure of the SiO ₂ layer 54 and the same thickness of the SiO ₂ layer 54 'formed on the surface of the supporting substrate 51 is formed by thermal oxidation. Further, hydrogen ions are implanted to a predetermined depth from the surface of the material wafer 52 through the SiO ₂ layer 54 ′ to form a hydrogen ion implanted layer 55.

Then, the surface of the SiO ₂ layer 54 ′ of the material wafer 52 on which the SiO ₂ layer 54 ′ and the hydrogen ion implantation layer 55 are formed and the surface of the Si layer 53 formed on the uppermost layer of the support substrate 51 are in close contact with each other at room temperature. Then, peeling is caused in the hydrogen ion implanted layer 55 by applying heat treatment. As a result, the SiO ₂ layer 54 ′ and the Si layer 53 ′ of the material wafer 52 are transferred to the support substrate 51 side, and a photonic crystal composed of the periodic structure 60 with three periods is formed on the support substrate 51.

Here, since the Si layer 53 'is a single crystal layer transferred from a silicon single crystal wafer, it has good crystallinity and can be suitably used as a liquid crystal peripheral circuit. Further, in order to improve the flatness of the surface of the Si layer 53 ′ immediately after peeling, it is preferable to perform planarization treatment by polishing, heat treatment, or a combination thereof, and in that case, the Si layer 53 after the planarization treatment is performed. The layer thickness of 'is matched with the layer thickness of the underlying Si layer 53.

In addition, although formation of Si layer 53 'in the said embodiment is thinning using what is called a smart cut method, it is not limited to this method, For example, grinding, polishing, an etching etc. are combined suitably. Then, the Si layer 53 ′ can be formed by thinning the material wafer 52 from the back surface (main surface opposite to the bonding surface).

Further, in the case where the outermost Si layer 53 on the support substrate 51 side to be the bonding surface is formed by CVD, the surface needs to be sufficiently flat in order to perform bonding well. Therefore, the surface of the outermost Si layer 53 is flattened by CMP (Chemical Mechanical Polishing) before bonding, or the surface of the SiO ₂ layer 54 before forming the Si layer 53 layer is flattened by CMP. It is preferable to add a technique such as depositing the Si layer 53 which is the outermost layer after the formation.

In addition, in the formation of the periodic structure having the three periods described above, the method of transferring the thin layer by hydrogen ion implantation (that is, the smart cut method) has been shown as an example used only for the process of forming the uppermost one period. The smart cut method can be repeated when the SiO ₂ layer 54 and the Si layer 53 are alternately formed on the support substrate 51 for two periods. In this case, since the peeling surface becomes a bonding surface of the next cycle, it is necessary to perform planarization treatment by polishing, heat treatment, or a combination of these. In this way, by repeating the smart cut method to form a desired periodic structure, a periodic structure with excellent layer thickness uniformity and flatness can be formed, so that the characteristics as a photonic crystal are extremely sufficient. It can be pulled out.

Peripheral circuits necessary for liquid crystal driving such as C-MOS or SRAM are formed in the element formation layer formed as described above. In this case, if a silicon single crystal wafer having a diameter of 200 mm or 300 mm is used as the support substrate, a normal LSI process can be applied as it is, and the cost of the substrate material is also advantageous. On the element formation layer of the multilayer substrate of the present invention thus produced, the same process as that of a conventional reflective liquid crystal display element is applied, and a liquid crystal layer, an ITO electrode, an alignment film, a transparent substrate (glass substrate), etc. By forming, the reflective liquid crystal display element of the present invention can be formed.

In the present invention, when the semiconductor element 104 as a peripheral circuit is formed in the element formation layer, the area occupied per pixel is made as small as possible as shown in FIG. 1, and the reflective film 109 is formed on the surface. The reflective film 109 can be formed using a material similar to the conventional one, for example, aluminum, silver, or a dielectric multilayer film. With such a structure, visible light (see FIG. 1D) incident on the semiconductor element 104 can only have a reflectance of about 90 to 96%, which is equivalent to the conventional one. Visible light (see FIG. 1C) incident on a portion other than the semiconductor element 104 occupying the area exhibits the performance of the underlying photonic crystal and can obtain a reflectivity of almost 100%, resulting in reflection. The performance of the liquid crystal display device 100 can be improved. FIG. 1 (a) schematically shows a case where incident light does not reach the photonic crystal due to the orientation of the liquid crystal, and FIG. 1 (b) shows a part of the incident light reaching the photonic crystal. The case is shown schematically. Even when only a part of incident light reaches the photonic crystal as shown in FIG. 1B, the reflectance by the photonic crystal is almost 100%.

Moreover, in the reflective liquid crystal display element of the present invention, a remarkable effect is to suppress the temperature rise of the liquid crystal. That is, in the conventional reflective liquid crystal display device as shown in FIG. 2, the infrared light (infrared rays) incident together with the visible light is absorbed by the reflective electrode or the metal wiring under the liquid crystal, so that the liquid crystal temperature is likely to rise. It was necessary to consider a cooling mechanism to prevent this. However, the photonic crystal used in the reflective liquid crystal display element of the present invention can increase the reflectance of visible light required for the liquid crystal display function to the ultimate, and is unnecessary for the liquid crystal display function. Infrared light is transmitted. Therefore, since the temperature rise of the liquid crystal is suppressed, it is not necessary to consider the cooling mechanism as in the prior art, and it is possible to prevent the apparatus from becoming large.

In the above embodiment, the Si layer has the maximum refractive index with respect to visible light, and the SiO ₂ layer has the minimum refractive index. However, the present invention is not limited thereto. For example, as a high refractive index material suitable for a layer having a maximum refractive index with respect to visible light,
Single elements such as Si, Ge, Be, Sb, Cr, Mn, and compounds such as 6h-SiC, 3c-SiC, BP, AlP, AlAs, AlSb, Sb ₂ S ₃ , GaP, ZnS, TiO _2, etc. Can be mentioned. All of these high refractive index material groups have a refractive index of 2.4 or more for visible light, but have high translucency for visible light, that is, low light absorption effect for visible light, Si, A material group consisting of 6h-SiC, 3c-SiC, BP, AlP, AlAs, GaP, ZnS, and TiO ₂ is particularly suitable. Furthermore, a material group made of Si, 6h-SiC, BP, AlP, AlAs, and GaP having a refractive index of 3.0 or more is particularly suitable. Among these, Si is the most suitable material because it is relatively inexpensive and can be easily formed into a thin film and its refractive index is as high as 3.5.

On the other hand, as a low refractive index material suitable for a layer having a minimum refractive index with respect to visible light, single elements such as Mg, Ca, Sr, Ba, Ni, Cu, Al, Au, Ag, and SiO ₂ , CeO ₂ , ZrO ₂ , MgO, Sb ₂ O ₃ , BN, AlN, Si ₃ N ₄ , Al ₂ O ₃ , TiN, CN and the like can be exemplified. All of these low refractive index material groups are composed of materials having a refractive index smaller than 2.2. However, when combined with the above high refractive index material, the refractive index difference becomes large. In particular, the refractive index difference is 1.0. It is desirable to appropriately select the above. Further, among the low refractive index material group, SiO ₂ , CeO ₂ , ZrO ₂ , MgO, Sb ₂ O ₃ , BN, AlN, Si ₃ N ₄ , Al ₂ O have a low light absorption effect on visible light. _A material group consisting of ₃ is particularly suitable for the medium. Furthermore, SiO ₂ having a low refractive index of 1.5 can be said to be the most suitable material.

Two silicon single crystal wafers having a diameter of 200 mm are prepared, a 103 nm SiO ₂ layer is formed on the surface of the first wafer by thermal oxidation, and hydrogen ions are implanted from the surface of the SiO ₂ layer. An ion implantation layer was formed at a depth of 100 nm from the surface.
Next, the surface of the SiO ₂ layer of the ion-implanted wafer and the surface of the second silicon single crystal wafer are bonded to each other, and heat treatment is performed and the ion-implanted layer is peeled off. _Two layers and a single crystal Si layer were transferred. Then, the surface (peeled surface) of the transferred single crystal Si layer was polished, and the thickness was adjusted to about 35 nm.

Next, a silicon single crystal wafer having a diameter of 200 mm is prepared separately, and after the formation of the SiO ₂ layer and the hydrogen ion implantation layer as described above, the single crystal Si adjusted to about 35 nm on the surface of the second wafer. The SiO ₂ layer and the single crystal Si layer in the second period were peeled off together with the layer surface, and the thickness of the peeled single crystal Si layer was adjusted to about 35 nm. Further, by repeating these steps, a three-period periodic structure having one cycle of a 103 nm SiO ₂ layer and a 35 nm single crystal Si layer was produced on the surface of the second silicon single crystal wafer.

The reflection spectrum was measured by making light incident on this surface, and the results are shown in FIG. According to FIG. 5, a so-called photonic band gap phenomenon is obtained in which light is not able to travel over a wide range of visible light (400 to 800 nm) but is totally reflected near a wavelength of 450 nm. I found out. In particular, it was confirmed that a high reflectance of 99% or more was obtained in the wavelength band of 550 nm to 700 nm.

It is a cross-sectional model diagram which shows an example of the multilayer substrate of this invention, and a reflection type liquid crystal display element using the same. It is a cross-sectional schematic diagram of a conventional reflective liquid crystal element. It is a figure which shows the flow which produces the element formation layer of the multilayer substrate of this invention. It is a schematic diagram for demonstrating the periodic structure in this invention. It is a figure which shows the reflection spectrum in the Example of this invention.

Explanation of symbols

100, 200 Reflective liquid crystal display element 101 Support substrate 102 Periodic structure 103 Element formation layer 104, 202 Device 105, 205 Liquid crystal layer 106, 206 Alignment film 107, 207 ITO electrode 108 Transparent substrate 201 Silicon substrate 203 Metal wiring 204 Reflective electrode 205 Liquid crystal layer 208 Glass substrate 10 High refractive index layer 11 Low refractive index layer 51, 52 Silicon single crystal wafer 53, 53 ′ Si layer 54, 54 ′ SiO ₂ layer
60 Periodic structure

Claims (9)

A multilayer substrate having at least a support substrate, a photonic crystal formed on the surface of the support substrate, and an element formation layer formed on the surface of the photonic crystal to produce a semiconductor element, The nick crystal is a periodic structure in which two or more layers having different refractive indexes with respect to visible light are periodically arranged, and the periodic structure is a one-dimensional photonic crystal with respect to the visible light. The multilayer substrate is characterized in that the layer thickness of one period is adjusted. The refractive index difference between the layer having the maximum refractive index and the layer having the minimum refractive index with respect to the visible light in each layer constituting one cycle of the periodic structure is 1.0 or more. 1. A multilayer substrate described in 1. In each layer constituting one period of the periodic structure, the Si layer has the maximum refractive index with respect to the visible light, and the SiO ₂ layer has the minimum refractive index. Item 3. The multilayer substrate according to Item 2. The multilayer substrate according to any one of claims 1 to 3, wherein the periodic structure includes two types of layers, and the number of periods is five or less. The multilayer substrate according to any one of claims 1 to 4, wherein the element formation layer is made of single crystal silicon. 6. The periodic structure according to claim 1, wherein the periodic structure has a reflectance of 98% or more in a wavelength band of at least 100 nm in the entire wavelength region of the visible light. Multilayer board. The multilayer substrate according to claim 1, wherein the support substrate is a single crystal silicon substrate. 8. The semiconductor device according to claim 1, wherein a semiconductor element is formed in a part of the element formation layer, and a reflective layer is formed on an outermost surface of a region where the semiconductor element is formed. A multilayer substrate according to one item. 9. The reflective liquid crystal display element according to claim 8, wherein the reflective element is a reflective liquid crystal display element having a structure in which at least a liquid crystal layer and a transparent substrate are laminated on an element forming layer on which the semiconductor element is formed. Controls the liquid crystal drive of the liquid crystal layer.

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