JP2016061806A - Optical filter and imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve wavelength resolution of an optical filter.SOLUTION: An optical filter 10 transmits light at a specific wavelength and includes a substrate 1 and a first conductor film 4, a dielectric film 3 and a second conductor film 2 layered on the substrate 1. The dielectric film 3 is disposed between the first conductor film 4 and the second conductor film 2. Openings 5 are periodically arranged in the first conductor film 4. The arrangement period of the openings 5 is less than the specific wavelength.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical filter that transmits light of a specific wavelength.

  Many substances have a characteristic absorption wavelength and fluorescence wavelength with respect to light irradiation. For this reason, the component of a substance can be identified by irradiating a substance with light with an irradiation apparatus and selectively detecting a specific wavelength of light from the substance using a spectroscopic apparatus. Thereby, much information regarding the object can be obtained.

  An example of a general-purpose spectroscopic camera is a hyperspectral camera. As a spectroscopic method, there are a method using a slit and a spectroscopic optical system, a method using an acousto-optic element, a method using a liquid crystal rear filter, and the like. By controlling the spectroscopic system, any wavelength can be extracted and imaged with a wavelength resolution of about 10 nm. Such a spectroscopic camera is very expensive because it uses a complicated optical system, and is limited to commercial use.

  On the other hand, if the camera does not have a spectroscopic device as described above and is focused on a specific application and only a specific wavelength is required, the specific wavelength is extracted by adding a multilayer filter to the camera. be able to. For example, in the following Non-Patent Documents 1 and 2, a foundation applied to the skin by attaching a multilayer optical filter having transmission characteristics at 560 nm to 610 nm, which is an absorption peak of hemoglobin contained in human blood, to a normal camera. The smears are imaged. In the case of using such a multilayer filter to see the wavelength in a fixed manner, it is necessary to change the filter for each purpose, which may cause a problem in convenience.

  As a camera having a spectroscopic function with improved convenience, for example, a camera using a metal film filter that transmits a specific wavelength shown in Patent Document 1 listed below can be cited as a candidate. This metal film filter has a metal film and an array of openings provided in the metal film. The apertures in the array are selected according to the wavelength of the incident light to the array so that incident light of a predetermined wavelength perturbs the metal film in the surface plasmon energy band and enhances the transmission of light passing through each aperture of the array Are arranged in a cycle. Non-patent documents 3 and 4 below also describe examples of metal film filters based on surface plasmon resonance.

Japanese Patent Laid-Open No. 11-72607 Ken Nishino et al., “Optical filter for highlighting spectral features Part I” OPTICS EXPRESS, 17 March 2011, Vol.19, No.7, pp.6020-6030 (2011) Ken Nishino et al., “Optical filter for highlighting spectral features Part II” OPTICS EXPRESS, 17 March 2011, Vol.19, No.7, pp.6031-6041 (2011) Naoki Ikeda et al., “Color Filter Based on Surface Plasmon Resonance Utilizing Sub-Micron Periodic Hole Array in Aluminum Thin Film” IEICE TRANSACTIONS on Electronics, Vol.E95-C No.2, pp.251-254 (2012) Ting Xu et al., “Plasmonic nanoresonators for high-resolution color filtering and spectral imaging” Nature Communications, 24 August 2010, Vol.1, 1058 (2010)

  In the metal film filters as described in Patent Document 1 and Non-Patent Documents 3 and 4, an electron wave called surface plasmon is generated on the surface of the metal film filter by incident light, and is present due to a periodic aperture disposed in the metal film filter. By selecting the wavelength of the electron wave, the transmission wavelength of the metal film filter is determined. Therefore, the half width of the transmission wavelength spectrum is mainly determined by the ratio between the aperture diameter and the period, the film thickness, the optical characteristics of the constituent materials, and the like. In the structures of Patent Document 1 and Non-Patent Documents 3 and 4, it is difficult to narrow the half bandwidth. For example, the half-value width of the transmission wavelength spectrum of the metal film filters described in Non-Patent Documents 3 and 4 is 100 nm to 200 nm. On the other hand, in order to detect an absorption peak of a specific substance, a wavelength resolution of 10 to 50 nm may be required as shown in Non-Patent Documents 1 and 2 above. In this case, it becomes difficult to obtain the required wavelength resolution from one optical filter. Therefore, the present application discloses an optical filter with improved wavelength resolution.

  An optical filter according to an embodiment of the present invention is an optical filter that transmits light of a specific wavelength, and includes a substrate and a first conductor film, a dielectric film, and a second conductor film stacked on the substrate. Prepare. The dielectric film is provided between the first conductor film and the second conductor film. Openings are periodically arranged in the first conductor film. The period in which the openings are arranged has a length shorter than the specific wavelength.

  According to the present disclosure, the wavelength resolution of the optical filter can be improved.

FIG. 1 is a perspective view illustrating a configuration example of an optical filter according to the first embodiment. FIG. 2 is a side perspective view of the optical filter shown in FIG. 3 is a cross-sectional view taken along line AA shown in FIG. FIG. 4 is a diagram illustrating an example in which openings are arranged in a regular triangular lattice shape. FIG. 5 is a diagram illustrating an example in which openings are arranged in a square lattice pattern. FIG. 6 shows the measurement results of the transmission characteristics of the optical filter of the present embodiment and the optical filter having no second conductor film as a comparative example. FIG. 7 shows the measurement results of the transmission characteristics when the thickness of the dielectric film 3 of the optical filter 10 is changed from 450 nm to 600 nm. FIG. 8 shows the change in transmittance at the maximum transmission wavelength when the thickness of the dielectric film 3 of the optical filter 10 is changed from 450 nm to 600 nm. FIG. 9 shows the relationship between the thickness of the dielectric film and the maximum transmission wavelength. FIG. 10 shows the transmission characteristics when the Al film thickness of the first conductor film 4 of the optical filter 10 is changed from 20 nm to 200 nm. FIG. 11 shows a change in transmittance at the maximum transmission wavelength when the Al film thickness of the first conductor film 4 of the optical filter 10 is changed from 20 nm to 200 nm. FIG. 12 is a diagram showing the transmission characteristics of an optical filter using only the first conductor film 4 without providing the second conductor film 2. FIG. 13 is a plot of the transmittance and half-value width of the main peak in the measurement results shown in FIG. FIG. 14 shows transmission characteristics when the Al film thickness of the second conductor film 2 of the optical filter 10 is changed from 5 nm to 10 nm. FIG. 15 is a side perspective view illustrating a configuration example of the optical filter 10a according to the second embodiment. 16 is a cross-sectional view taken along line AA in FIG. FIG. 17 shows the measurement results of the transmission characteristics of the optical filter 10a of the second embodiment and the transmission characteristics of the optical filter 10 of the first embodiment. FIG. 18 is a diagram showing the reflectance characteristics of Ag and Al. FIG. 19 is a side perspective view illustrating a configuration example of the optical filter 10b according to the fourth embodiment. 20 is a cross-sectional view taken along line AA in FIG. FIG. 21 is a diagram illustrating the transmission characteristics of the optical filter 10b according to the fourth embodiment. FIG. 22 is a diagram illustrating a configuration example of the image sensor 30 according to the fifth embodiment. FIG. 23 shows a light receiving element 20, an exploded perspective view of the optical filter 10, and an enlarged schematic view of the surface of the first conductor film 4 of the optical filter 10. FIG. 24 is a partial cross-sectional view showing a modification of the image sensor 30.

  An optical filter according to an embodiment of the present invention is an optical filter that transmits light of a specific wavelength, and includes a substrate and a first conductor film, a dielectric film, and a second conductor film stacked on the substrate. Prepare. The dielectric film is provided between the first conductor film and the second conductor film. Openings are periodically arranged in the first conductor film. The period in which the openings are arranged has a length shorter than the specific wavelength.

  According to the above configuration, when light is incident on the first conductor film, surface plasmons are excited on the surface of the first conductor film. The surface plasmon and incident light resonate, and light having a wavelength corresponding to the period of the opening is emitted. As a result, light near a specific wavelength selectively passes through the optical filter. Further, since light interference occurs between the first conductor film and the second conductor film, the wavelength of the transmitted light is further narrowed. As a result, the wavelength resolution of the optical filter is improved.

  A cross section of the opening in a plane perpendicular to the thickness direction of the first conductor film may be any one of a circle, an ellipse, and a polygon.

  In the first conductor film, the opening may be arranged in an isosceles triangular lattice, a regular triangular lattice, or a square lattice. Thereby, it is possible to suppress the incident polarization dependence and improve the oblique incidence characteristics.

  The plasma frequency of the first conductor film and the second conductor film may be higher than the frequency of the specific wavelength. For example, the first conductor film and the second conductor film can be formed of metal.

  The film thickness of the first conductor film may be in the range of 50 nm to 200 nm, and the film thickness of the second conductor film may be in the range of 5 nm to 50 nm. Thereby, the light of the wavelength selected by the resonance of the surface plasmon excited on the surface of the first conductor film and the incident light is easily reflected and transmitted by the second conductor film. As a result, the wavelength of the selected light is easily narrowed.

  The optical filter may be configured such that surface plasmons are induced on the surface of the first conductor film by light incident on the first conductor film. The surface plasmon and the incident light resonately interact with each other through the openings periodically arranged in the first conductor film, so that the transmitted light transmitted through the first conductor film is transmitted. The opening may be arranged so that the wavelength is limited to a region including the specific wavelength. The transmission light or the incident light may cause optical reflection, transmission, and interference between the first conductor film and the second conductor film.

  The first conductor film or the second conductor film may include at least one material selected from the group consisting of aluminum, silver, copper, gold, or an alloy thereof. The first conductor film and the second conductor film may be aluminum, or a mixture or alloy containing aluminum.

  The dielectric material and the material filling the opening of the first conductor film are the same material, and the material is at least selected from the group consisting of silicon oxide, silicon nitride, titanium oxide, and aluminum oxide. One can be included.

  The distance between the first conductor film and the second conductor film may be 0.5 to 4.0 times the specific wavelength. Thereby, it is possible to easily cause light interference between the first conductor film and the second conductor film.

  An imaging device including the optical filter described above and a light receiving element that receives light transmitted through the optical filter and converts the light into an electric signal is also an example of an embodiment of the present invention.

  In the imaging element, the light receiving element may include a plurality of pixels, and the optical filter may be provided corresponding to each pixel. In this case, the specific wavelength of light that can pass through an optical filter provided corresponding to one pixel among the plurality of pixels is different from the specific wavelength of light that can pass through an optical filter provided corresponding to another pixel. It can be configured. Thereby, for example, a color filter of a different color for each pixel can be formed with a simple structure.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. In addition, in order to make the explanation easy to understand, in the drawings referred to below, the configuration is shown in a simplified or schematic manner, or some components are omitted. Further, the dimensional ratio between the constituent members shown in each drawing does not necessarily indicate an actual dimensional ratio.

<Embodiment 1>
FIG. 1 is a perspective view illustrating a configuration example of an optical filter according to the first embodiment. FIG. 2 is a side perspective view of the optical filter shown in FIG. 3 is a cross-sectional view taken along line AA shown in FIG. The optical filter 10 transmits light having a specific wavelength. The specific wavelength can be a wavelength included in a wavelength band of light desired to be transmitted through the optical filter 10 or a wavelength band. The specific wavelength can be, for example, the wavelength of light having the highest transmittance among transmitted light, that is, the maximum transmission wavelength.

  In the example shown in FIG. 1, the optical filter 10 includes a substrate 1, a first conductor film 4, a second conductor film 2, and a dielectric film 3. In this example, the second conductor film 2, the dielectric film 3, and the first conductor film 4 are sequentially stacked on the substrate 1. The dielectric film 3 is provided between the first conductor film 4 and the second conductor film 2. That is, the first conductor film 4 and the second conductor film 2 sandwich the dielectric film 3. Openings 5 are periodically arranged in the first conductor film 4. The period P in which the opening 5 is arranged has a length shorter than the specific wavelength. The second conductor film 2 does not have an opening. The film thickness of the second conductor film 2 is smaller than the film thickness of the first conductor film 4. As shown in FIG. 2, a dielectric film 6 can be provided on the first conductor film 4 so as to cover the first conductor film 4. The dielectric film 6 is not shown in FIG.

  In this example, light is incident in the direction indicated by the arrow F in FIG. The first conductor film 4 is located upstream of the second conductor film 2 with respect to the light incident direction. Of the light incident on the first conductor film 4, light in a band including a specific wavelength is emitted from the first conductor film 4 to the second conductor film 2. A part of the light incident on the second conductor film 2 is transmitted through the second conductor film 2, and the other part is reflected by the second conductor film 2. Between the first conductor film and the second conductor film 2, light interference occurs, and the band of light transmitted through the second conductor film 2 is further narrowed. Therefore, light having a specific wavelength and having a narrower band than the light emitted from the first conductor film 4 passes through the second conductor film 2. A specific example will be described later.

  In the example shown in FIGS. 1 and 2, the opening 5 is a through-hole that penetrates the first conductor film 4, but a hole that does not penetrate the first conductor film can also be used as the opening. The first conductor film 4 can be periodically provided with a portion (for example, a portion whose film thickness is thinner than the other portion) that allows light to pass more easily than the other portion as the opening. On the other hand, the film thickness of the second conductor film 2 can be made uniform over the entire film. That is, the second conductor film 2 can be formed so that the light passage is substantially the same over the entire film.

  The material of the substrate 1 is not particularly limited as long as it is a material that transmits incident light. For example, any of an inorganic material, an organic material, or a mixed material thereof can be used as the material of the substrate 1. As a material of the substrate 1, for example, glass, quartz, Si, a compound semiconductor, or the like can be used. Further, the size and thickness of the substrate 1 are not particularly limited. The surface shape of the substrate is not particularly limited, and may be flat or curved. In consideration of adhesion with a layer formed on the substrate 1, a film may be laminated on the substrate 1 after performing an appropriate surface treatment on the substrate 1. Alternatively, a transparent material having high resistance to etching can be laminated on the substrate 1 as a stopper layer, and then the film can be laminated.

  The conductive material constituting the first conductor film 4 can be arbitrarily selected. For example, as the conductive material, a material made of a metal element which is a single conductor, has a reflectance of 70% or more in an arbitrary wavelength band, and is solid at room temperature, and an alloy thereof can be used. The plasma frequency of the material constituting the first conductor film 4 is preferably higher than the frequency of the light to be used, that is, the light transmitted through the optical filter 10. For example, a material having a plasma frequency lower than a specific wavelength can be used as the material of the first conductor film 4. In addition, it is desirable that light absorption is small in the wavelength region of light to be used. Such a material may include, for example, a material selected from the group consisting of aluminum, copper, silver, gold, titanium nitride, zirconium nitride, nickel, cobalt, and alloys thereof. Alternatively, an In 2 O 3 system including ITO (Sn: In 2 O 3), an AZO (Al: ZnO), a GZO (Ga: ZnO), a BZO (B: ZnO), or a ZnO system including IZO (In: ZnO), an IGZO system A material selected from metal oxide transparent conductive materials can be included in the first conductor film 4. In addition, the material of the 1st conductor film 4 is not restricted to the said example. Further, the first conductor film 4 may be sintered by heat treatment, and a protective film or the like may be formed on the first conductor film 4. The thickness of the first conductor film 4 is preferably 50 nm or more and 200 nm or less.

  A plurality of openings 5 are arranged in the first conductor film 4 with a period shorter than the wavelength of the specific wavelength. As shown in FIG. 4, the openings 5 are arranged such that the centers of the openings 5 are located at the vertices of the regular triangle (regular triangular lattice shape). Here, it may be arranged such that the center of the opening 5 is positioned at each vertex of the isosceles triangle instead of the regular triangle. That is, a plurality of rows of openings 5 arranged at equal intervals in the first direction (for example, the x direction) are arranged in the second direction (for example, the y direction) perpendicular to the first direction. The openings 5 in adjacent rows are shifted in the first direction so that the openings 5 in one row in the first direction are located at an equal distance from the two openings 5 in the adjacent row. In this way, by arranging the openings in a regular triangular lattice shape or an isosceles triangular lattice shape, it is possible to suppress the incident polarization dependence and improve the oblique incident characteristics. The period P in which the openings 5 are arranged is preferably, for example, 150 nm or more and 1000 nm or less.

  Note that the arrangement of the openings 5 is not particularly limited. For example, as shown in FIG. 5, the openings 5 may be arranged in a square lattice shape. Moreover, although the opening part 5 is cylindrical shape in this example, the shape is not limited to this. For example, the cross-sectional shape in a plane perpendicular to the penetrating direction of the opening 5 (the thickness direction of the first conductor film 4) can be a circle, an ellipse, a triangle, a rectangle, or other polygons. Further, the side wall (inner wall) of the opening 5 may be inclined with respect to the penetrating direction. For example, the opening 5 may have a conical shape, a triangular pyramid shape, a quadrangular pyramid shape, or the like. The opening 5 is filled with the same dielectric as the dielectric film 3.

  The dielectric material constituting the dielectric film 3 and the dielectric film 6 can be one kind of material. Examples of such a material include silicon oxide, silicon nitride, titanium oxide, and aluminum oxide.

  The material of the second conductor film 2 is less light-absorbing in the wavelength region of light to be used, like the first conductor film 4, and the plasma frequency of the second conductor film 2 is higher than the frequency of light to be used. It is preferable. The material of the second conductor film 2 may not be the same material as the first conductor film 4. In addition, the thickness of the second conductor film 2 can be set to, for example, 5 nm or more and 50 nm or less from the viewpoint of providing light reflection and transmission functions. The film thickness of the dielectric film 3 between the first conductor film 4 and the second conductor film 2 is, for example, from the viewpoint of causing light interference between the first conductor film 4 and the second conductor film 2. 150 nm or more and 5000 nm or less.

  In the optical filter 10 having the above configuration, incident light excites surface plasmons on the surface of the first conductor film 4, and the surface plasmons and incident light interact in a resonant manner. At this time, light in a band including a specific wavelength is emitted from the first conductor film 4 by resonance according to the period P of the opening 5. This light interferes between the second conductor film 2 and the first conductor film 4. Accordingly, the second conductor film 2 and the first conductor film 4 interfere with each other from the wavelength band of the light emitted by the resonance by the period P of the opening 5 of the first conductor film 4, and the second conductor film 4 The wavelength band of light transmitted through the conductor film 2 becomes narrower. That is, the light whose wavelength is more selected passes through the second conductor film.

  In the present embodiment, interference occurs between the first conductor film 4 and the second conductor film 2, so that the transmission wavelength is narrowed. In order to cause this interference, a plane wave is preferably formed between the first conductor film 4 and the second conductor film 2. From this point of view, the distance between the first conductor film 4 and the second conductor film 2 is that the light having a specific wavelength to be transmitted forms a plane wave between the first conductor film 4 and the second conductor film 2. It is desirable that the distance be equal to or greater than the minimum distance required for the operation. Therefore, for example, the distance between the first conductor film 4 and the second conductor film 2 can be a distance from 0.5 times to 4.0 times the maximum transmission wavelength.

  For manufacturing the optical filter 10, for example, a fine processing technique such as a photolithography method, an electron beam lithography method, or a nanoimprint method can be used. As a manufacturing method of the optical filter 10, for example, the second conductor film 2 is formed on the substrate 1. Next, the dielectric film 3 is laminated on the second conductor film 2. Subsequently, a first conductor film 4 is formed on the dielectric film 3, and an opening 5 is formed in the first conductor film 4 by photolithography and etching. Then, the dielectric film 6 is filled in the opening 5 and the dielectric film 5 is laminated on the first conductor film 4 to obtain the optical filter 10. In order to prevent problems such as side etching of the inner wall of the opening 6 of the first conductor film 4, processing can be performed under highly anisotropic dry etching conditions.

  FIG. 6 shows the measurement results of the transmission characteristics of the optical filter 10 of the present embodiment and the optical filter having no second conductor film as a comparative example. In the example shown in FIG. 6, the half-value width of the maximum transmitted light in the optical filter 10 provided with the second conductor film 2 is narrower than the half-value width of the maximum transmitted light of the optical filter not having the second conductor film. ing.

The optical filter 10 in the measurement of FIG. 6 includes a substrate 1 made of SiO ₂ having a thickness of 300 nm, a second conductor film 2 made of Al having a thickness of 5 nm, and a thickness made of SiO _2. A dielectric film 3 having a thickness of 500 nm, a first conductor film 4 having a thickness of 75 nm formed of Al, and a dielectric film 6 having a thickness of 300 nm formed of SiO ₂ . The opening 5 has a cylindrical pattern arranged in a triangular lattice pattern, the period P is 360 nm, and the diameter D is 180 nm.

The optical filter of the comparative example includes an 800 nm thick substrate formed of SiO ₂ , a 75 nm thick first conductor film formed of Al, and a 300 nm thick dielectric film formed of SiO _2. Is provided. The openings have the same pattern as in this embodiment, the period is 360 nm, and the diameter is 180 nm.

  Next, the evaluation result of the film thickness of the dielectric film 3 of the optical filter of the first embodiment is shown. FIG. 7 shows the measurement results of the transmission characteristics when the thickness of the dielectric film 3 of the optical filter 10 of this embodiment is changed from 450 nm to 600 nm. FIG. 8 shows the change in transmittance at the maximum transmission wavelength when the thickness of the dielectric film 3 of the optical filter 10 is changed from 450 nm to 600 nm. In these examples, the maximum transmission wavelength of the optical filter 10, the half width of the maximum transmission wavelength, and the transmittance of the maximum transmission wavelength depend on the film thickness of the dielectric film 3.

FIG. 9 shows the relationship between the thickness of the dielectric film and the maximum transmission wavelength. In the example shown in FIG. 9, the thickness of the dielectric film and the maximum transmission wavelength are in a substantially proportional relationship. The transmission wavelength λ due to interference can be expressed by the following equation (1). According to the following formula (1), the dielectric film thickness and the transmission wavelength are in a proportional relationship. From this, it can be seen that interference occurs between the first conductor film and the second conductor film in the example shown in FIG.
λ = m × (2 · n · d) (1)
In the above formula (1), m = order, n = refractive index of the dielectric, d = dielectric film thickness, and λ = transmission wavelength.

  In the example shown in FIG. 9, the film thickness of the dielectric film is about 0.96 times the maximum transmission wavelength. That is, the film thickness of the dielectric film 3 provided between the first conductor film 4 and the second conductor film 2 is in the range of 0.5 to 4.0 times the maximum transmission wavelength. As described above, by setting the film thickness of the dielectric film 3, the first conductive film 4 and the second conductive film are generated so that a plane wave is more reliably generated between the first conductive film 4 and the second conductive film 2. A distance between the two can be ensured. In the example shown in FIGS. 7 to 9, if the distance between the first conductive film 4 and the second conductive film 2 is about 50 nm or more, a plane wave is generated between the first conductive film 4 and the second conductive film 2. I can guess that.

  Next, the evaluation result of the film thickness of the 1st conductor film 4 of the optical filter 10 of this embodiment is shown. FIG. 10 shows the transmission characteristics when the Al film thickness of the first conductor film 4 of the optical filter 10 is changed from 20 nm to 200 nm. FIG. 11 shows a change in transmittance at the maximum transmission wavelength when the Al film thickness of the first conductor film 4 of the optical filter 10 is changed from 20 nm to 200 nm. In the example shown in FIGS. 10 and 11, the maximum transmission wavelength and the transmittance of the maximum transmission wavelength of the optical filter 10 depend on the film thickness of the first conductor film 4. In the example shown in FIG. 11, when the film thickness of the first conductor film 4 is 20 nm or more and 200 nm or less, the transmittance at the maximum transmission wavelength is at an acceptable level. In particular, the transmittance is high when the film thickness is 50 nm or more and 150 nm or less.

FIG. 12 is a diagram showing the transmission characteristics of an optical filter using only the first conductor film 4 without providing the second conductor film 2. FIG. 12 shows the transmission characteristics when the thickness of the first conductor film 4 is changed between 20 nm and 400 nm. Optical filter in the measurement of Figure 12, on a substrate 1 having a thickness of 800nm formed by SiO _2, the first conductor film 4 formed by Al, dielectric thickness 300nm was formed of SiO ₂ A membrane 3. The openings 5 of the second conductor film 4 are cylindrical patterns arranged in a triangular lattice shape, the period P is 360 nm, and the diameter D is 180 nm.

  FIG. 13 plots the transmittance and half-value width of the main peak (peak at the maximum transmission wavelength) in the measurement results shown in FIG. 12 for each film thickness. The square plot in FIG. 13 represents the transmittance of the main peak, and the triangular plot represents the half width. In the example shown in FIG. 13, when the thickness of the first conductor film 4 is smaller than 50 nm, the transmittance of the main peak tends to decrease, and the full width at half maximum increases rapidly. When the film thickness is larger than 200 nm, the full width at half maximum hardly decreases, but the degree of decrease in transmittance increases. In this example, when the thickness of the first conductor film is in the range of 50 nm to 200 nm, the half width is relatively narrow and the transmittance is relatively high.

  Next, the result of having evaluated the film thickness of the 2nd conductor film 2 of the optical filter 10 of this embodiment is shown. FIG. 14 shows transmission characteristics when the Al film thickness of the second conductor film 2 of the optical filter 10 is changed from 5 nm to 10 nm. In the example shown in FIG. 14, when the thickness of the second conductor film 2 is increased, the transmittance is lowered, but a significant change in the wavelength of interference, that is, no significant change in the maximum transmission wavelength is observed.

<Embodiment 2>
FIG. 15 is a side perspective view illustrating a configuration example of the optical filter 10a according to the second embodiment. 16 is a cross-sectional view taken along line AA in FIG. In the optical filter 10a, a first conductor film 4a, a dielectric film 3, a second conductor film 2a, and a dielectric film 6 are sequentially stacked on the substrate 1. That is, FIG. 15 has a configuration in which the first conductor film and the second conductor film are interchanged in the layer configuration shown in FIGS. 1 and 2. Here, the 2nd conductor film 2a is arrange | positioned upstream with respect to the incident direction of light from the 1st conductor film 4a. Even in this configuration, the same effect as in the first embodiment can be obtained. The materials, shapes, and dimensions of the first conductor film 4a, the dielectric film 3, the second conductor film 2a, and the dielectric film 6 can be the same as those in the first embodiment.

  In the optical filter 10a having the configuration shown in FIGS. 15 and 16, the incident light is partially transmitted through the second conductor film 2a, and the light interfered between the second conductor film 2a and the first conductor film 4a is transmitted. Then, surface plasmon is induced on the surface of the first conductor film 4a. The surface plasmon and the incident light interact with each other in a resonant manner, and a predetermined wavelength is emitted by resonance due to the period of the opening 5. In this example, interference occurs between the light reflected by the first conductor film 4a having the opening 5 and the incident light transmitted through the second conductor film 2a.

  As a manufacturing method of the optical filter 10a, for example, first the first conductor film 4a is first formed on the substrate 1. Subsequently, the opening 5 is formed by photolithography and etching. Next, the dielectric film 3 is filled in the opening 5 and laminated on the first conductor film 4a, and is planarized by a chemical or physical planarization technique. Subsequently, the second conductor film 2 a is laminated on the dielectric film 3. Subsequently, the dielectric film 6 is laminated on the second conductor film 2a to obtain the optical filter 10a. In order to prevent problems such as side etching of the inner wall of the opening 5 of the first conductor film 4a, it can be processed under highly anisotropic dry etching conditions.

  FIG. 17 shows the measurement results of the transmission characteristics of the optical filter 10a of the second embodiment and the transmission characteristics of the optical filter 10 of the first embodiment. In the example illustrated in FIG. 17, the half-value width and the transmittance at the maximum transmission wavelength of the optical filter 10 a according to the second embodiment are substantially the same as those of the optical filter 10 according to the first embodiment. In this example, the same effect can be obtained regardless of whether the conductor layer 4 including the openings 5 arranged periodically is arranged upstream or downstream of the conductor layer 2 not including the openings 5.

Note that the optical filter 10a of the second embodiment in the measurement shown in FIG. 17 includes a substrate 1 made of SiO ₂ with a thickness of 300 nm, a first conductor film 4a made of Al with a thickness of 75 nm, and SiO _2. The dielectric film 3 having a thickness of 500 nm formed by the above, the second conductive film 2a having a thickness of 10 nm formed by Al, and the dielectric film 6 having a thickness of 300 nm formed by SiO ₂ are provided. The opening 5 has a cylindrical pattern arranged in a triangular lattice pattern, the period P is 360 nm, and the diameter D is 180 nm. The optical filter 10 of Embodiment 1 in the measurement shown in FIG. 17 has a configuration in which the arrangement of the first conductor film 4a and the second conductor film 2a of the optical filter 10a is switched up and down.

<Embodiment 3>
The optical filter of Embodiment 3 has a structure in which the material of the second conductor film 2a of Embodiment 2 is changed from Al to Ag. Ag has a higher reflectance than Al in at least the visible light region (see FIG. 18). Therefore, Ag is effective as a material for the interference film. That is, in order to easily cause interference, it is effective to form the first conductor film 4a and the second conductor film 2a with Ag. On the other hand, since the first conductor film 4a is subjected to fine processing of a periodic pattern (opening 5) by a semiconductor process, the material of the first conductor film 4a is a material having an affinity with the semiconductor process. It is preferable to use it.

Therefore, in the third embodiment, as an example, the substrate 1 made of SiO ₂ with a thickness of 300 nm, the first conductor film 4a made of Al with a thickness of 75 nm, and the thickness made of SiO ₂ with a thickness of 500 nm. The optical filter includes the dielectric film 3, the second conductor film 2 a having a thickness of 10 nm formed of Ag, and the dielectric film 3 having a thickness of 300 nm formed of SiO ₂ . The opening 5 has a cylindrical pattern arranged in a triangular lattice shape, and its period P can be 360 nm and its diameter D can be 180 nm. This makes it easier to cause interference and facilitates processing in the semiconductor process. The same effect can be obtained even in the structure in which the second conductor film 2a of the first embodiment is changed from Al to Ag.

<Embodiment 4>
FIG. 19 is a side perspective view illustrating a configuration example of the optical filter 10b according to the fourth embodiment. 20 is a cross-sectional view taken along line AA in FIG. In the optical filter 10 b, the sub-peak cut filter 7 is further laminated on the substrate 1. The optical filter 10b has a configuration in which the sub-peak cut filter 7 is provided on the dielectric film 6 in the optical filter 10a shown in FIG. The sub-peak cut filter 7 can be an additional filter film that passes a band including a specific wavelength that is transmitted by the optical filter 10d. The sub-peak cut filter 7 can include, for example, an organic film or a multilayer film.

The sub-peak cut filter 7 can be an organic film filter or a multilayer filter that transmits one of a plurality of predetermined wavelengths that are transmitted by the optical filter 10b. As the sub-peak cut filter 7, for example, an organic film filter such as an organic material containing pigment pigment or dye pigment, or a high refractive index material such as SiN, TiO ₂ or ZnS and a low refractive index such as SiO ₂ or MgF _{2 is used.} A multilayer interference film filter in which materials are alternately stacked can be used. Note that the wavelength of transmitted light of the sub-peak cut filter 7 is appropriately designed.

  FIG. 21 is a diagram illustrating the transmission characteristics of the optical filter 10b according to the fourth embodiment. A solid line indicates the transmission characteristics of the optical filter 10a of the second embodiment in the optical filter 10b of the fourth embodiment, and a broken line indicates the transmission characteristics of the sub-peak cut filter 7 in the optical filter 10b of the fourth embodiment. Here, as an example, a filter that transmits light of about 460 to 580 nm is used as the sub-peak cut filter 7. The sub-peak cut filter 7 transmits light having a peak of about 520 nm in the optical filter 10b portion of Embodiment 2, and does not transmit light having peaks of about 400 nm or less and about 600 nm or more. As a result, in the optical filter 10b of this embodiment, only light having a peak of about 520 nm is transmitted.

  Thus, since the optical filter 10b of this embodiment includes the sub-peak cut filter 7 as an additional filter film, it is possible to remove unnecessary wavelengths and transmit only desired wavelengths. Therefore, an optical filter having a desired resolution can be obtained.

<Embodiment 5>
FIG. 22 is a diagram illustrating a configuration example of the image sensor 30 according to the fifth embodiment. The imaging element 30 includes an optical filter 10 and a light receiving element 20 that receives light transmitted through the optical filter 10 and converts it into an electrical signal. The optical filter 10 can have the same configuration as the optical filter of any one of the first to fourth embodiments. The light receiving element 20 includes a plurality of pixels 20-1 to 20-4, and optical filters 10-1 to 10-4 are provided corresponding to the respective pixels. Here, the pixel 20-1 and the optical filter 10-1 corresponding to the pixel 20-1 are arranged side by side in a direction perpendicular to the imaging surface. Although not shown, the imaging element 30 can further include a signal processing circuit that calculates an electrical signal from the light receiving element.

  Here, the wavelength bands of the transmitted light of the optical filters 10-1 to 10-4 may be different. That is, the transmission wavelength of the optical filters 10-1 to 10-4 corresponding to each pixel can be set as necessary. For example, the transmission wavelength of the optical filter corresponding to each pixel can be set so that light of a different wavelength can be detected for each pixel.

  For example, an optical filter 10 having a metal thin film that selectively transmits wavelengths from the ultraviolet to the infrared region is manufactured, and the optical filter 10 is disposed on the pixel of the light receiving element 20 so that the spectroscope integrated type can be obtained. A spectral imaging device can be formed. Thereby, a spectral image can be taken with one chip. The period of the opening pattern required as an optical filter in the visible light region is in the range of 100 nm to 1000 nm that can be easily processed by the current semiconductor processing technology. Therefore, the optical filter can be directly formed on the CCD or CMOS imaging device in the semiconductor manufacturing process. Furthermore, since a periodic pattern of a plurality of optical filters can be formed by a single lithographic process, an optical filter having different wavelength selectivity for each pixel of the imaging device can be easily formed. Therefore, the manufacture of a multispectral camera can be facilitated.

  FIG. 23 shows a light receiving element 20, a perspective view of the optical filter 10, and a schematic diagram in which the surface of the first conductor film 4 of the optical filter 10 is enlarged. A plurality of pixels are arranged in a lattice shape (matrix shape) on the light receiving surface of the light receiving element 20. The optical filter 10 is also arranged in a matrix so as to correspond to each pixel of the light receiving element 20. The optical filter 10 and the light receiving element 20 are disposed so as to overlap in the light incident direction F. Each optical filter 10 includes the first conductor film 4 including the opening 5 as shown in the first to fourth embodiments. The period P of the opening 5 can be set as necessary for each optical filter 10 corresponding to each pixel.

  FIG. 24 is a partial cross-sectional view showing a modification of the image sensor 30. FIG. 24 shows a portion of one pixel in the image sensor 30. On the silicon substrate 16, a light receiving element 20a, an electrode 14, a light shielding film 13, an optical filter 10, a planarizing layer 12, and a microlens 11 are disposed. In this configuration, by providing the optical filter according to any one of the first to fourth embodiments as the optical filter 10, it is possible to obtain spectral imaging elements having different light receiving wavelengths for each pixel. In order to set the light reception wavelength for each pixel, the period P of the opening 5 of the first conductor film 4 may be adjusted as in the first to fourth embodiments.

  According to the optical filters of Embodiments 1 to 4, it is possible to improve the wavelength resolution of a filter using a conductor film having a periodic pattern. Therefore, the optical filter can be applied to a camera having a spectroscopic function having various uses.

  For example, substances originating from organisms such as animals, plants, and microorganisms have many absorption or emission wavelengths specific to the near ultraviolet to visible and near infrared regions. Therefore, it is possible to obtain various life information such as human health, food freshness and contamination, and can be widely applied in health, beauty, food management and the like. In particular, when a spectroscopic optical system using the optical filter is applied to a camera to image information of a specific wavelength, more intuitive and useful information for life can be obtained.

In addition, application of the optical filter in the said embodiment is not restricted to the camera with said spectral function. For example, in addition to the camera of the two-dimensional light receiving sensor, the above optical filter can be applied to a one-dimensional light receiving element. For example, water retention sensors, body fat sensors, CO2 sensors, proximity sensors, ambient light sensors, UV sensors, or line spectrum sensors (for Raman scattering spectroscopy, dispersion type and Fourier transform type infrared absorption spectroscopy), etc. The optical filter in the above embodiment can be applied.

DESCRIPTION OF SYMBOLS 1 Substrate 2, 2a 2nd conductor film 3, 6 Dielectric film 4, 4a 1st conductor film 5 Opening part 7 Subpeak cut filter 10, 10a, 10b Optical filter

Claims (13)

An optical filter that transmits light of a specific wavelength,
A substrate,
A first conductor film, a dielectric film and a second conductor film laminated on the substrate;
The dielectric film is provided between the first conductor film and the second conductor film,
Openings are periodically arranged in the first conductor film,
The period in which the openings are arranged is an optical filter having a length shorter than the specific wavelength.   2. The optical filter according to claim 1, wherein a cross section of the opening in a plane perpendicular to the thickness direction of the first conductor film is any one of a circle, an ellipse, and a polygon.   3. The optical filter according to claim 1, wherein in the first conductive film, the opening is arranged in an isosceles triangular lattice, a regular triangular lattice, or a square lattice.   The optical filter according to any one of claims 1 to 3, wherein a plasma frequency of the first conductor film and the second conductor film is higher than a frequency of the specific wavelength.   The optical filter according to claim 1, wherein the first conductor film and the second conductor film are formed of metal. A thickness of the first conductor film is in a range of 50 nm to 200 nm;
The optical filter according to any one of claims 1 to 5, wherein a film thickness of the second conductor film is in a range of 5 nm to 50 nm. Surface plasmon is induced on the surface of the first conductor film by light incident on the first conductor film,
The surface plasmon and the incident light resonately interact with each other through the openings periodically arranged in the first conductor film, so that the wavelength of the transmitted light transmitted through the first conductor film is reduced. The opening is disposed so as to be limited to a region including the specific wavelength,
The optical filter according to claim 1, wherein the transmitted light or the incident light causes optical reflection, transmission, and interference between the first conductor film and the second conductor film.   The first conductor film or the second conductor film includes at least one material selected from the group consisting of aluminum, silver, copper, gold, or an alloy thereof. The optical filter according to item 1.   The optical filter according to claim 1, wherein the first conductive film and the second conductive film are aluminum, a mixture or an alloy containing aluminum.   The dielectric material and the material filling the opening of the first conductor film are the same material, and the material is at least selected from the group consisting of silicon oxide, silicon nitride, titanium oxide, and aluminum oxide. The optical filter according to claim 1, comprising one.   The optical filter according to any one of claims 1 to 10, wherein a distance between the first conductor film and the second conductor film is 0.5 to 4.0 times the specific wavelength.   An image pickup device comprising: the optical filter according to claim 1; and a light receiving element that receives light transmitted through the optical filter and converts the light into an electric signal. The light receiving element includes a plurality of pixels,
The optical filter is provided corresponding to each pixel,
The specific wavelength of light that can pass through an optical filter provided corresponding to one pixel among the plurality of pixels is different from a specific wavelength of light that can pass through an optical filter provided corresponding to another pixel. The imaging device according to 12.

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