1. Field
The present disclosure relates to spectral devices, more specifically, a spectral device including a slit optical filter that includes a metal layer in which multiple slits are formed at a predetermined pitch, the optical filter transmitting light, most of which falls within a predetermined wavelength range.
2. Description of the Related Art
In recent years, optical filters (slit optical filters) that include a metal layer in which multiple slits are formed at a predetermined pitch to transmit light, most of which falls within a predetermined wavelength range, have been developed. An example of slit optical filters has been disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-525863.
Examples of factors that function as noises during use of optical filters include reflected waves, intervening light from adjacent pixels, light unintendedly leaking out from a gap, and unintended resonant waves. Noises affect spectral characteristics of an object and render true spectral characteristics unknown, which is a problem for an image-pickup device (for example, multispectral camera) including a spectral device having a narrow selective wavelength range. Moreover, in such slit optical filters, full width at half maximum (FWHM) is unintentionally increased by unintended resonant waves or reflected waves.
It is desirable to improve the light transmittance of a spectral device including a slit optical filter that includes a metal layer in which multiple slits are formed at a predetermined pitch, the optical filter transmitting light, most of which falls within a predetermined wavelength range.
According to an aspect of the disclosure, a spectral device includes a polarizing filter and an optical filter. The polarizing filter transmits part of light incident on the polarizing filter, the part of light having a particular polarization component. Light that is incident on and passes through the polarizing filter is converted into linearly polarized light. Light that has passed through the polarizing filter is incident on the optical filter. The optical filter transmits light within a particular frequency range. The optical filter includes a metal layer and a dielectric layer. The dielectric layer is disposed on the metal layer. Multiple slits are formed in the metal layer. The multiple slits are arranged at equal intervals in a predetermined direction. The multiple slits extend in a direction perpendicular to a direction in which light that has passed through the polarizing filter is polarized.
A spectral device according to an aspect of the disclosure can have higher light transmittance.
A spectral device according to one embodiment of the disclosure includes a polarizing filter and an optical filter. The polarizing filter transmits part of light incident on the polarizing filter, the part of light having a particular polarization component. Light that is incident on and passes through the polarizing filter is converted into linearly polarized light. Light that has passed through the polarizing filter is incident on the optical filter. The optical filter transmits light within a particular frequency range. The optical filter includes a metal layer and a dielectric layer. The dielectric layer is disposed on the metal layer. Multiple slits are formed in the metal layer. The multiple slits are arranged at equal intervals in a predetermined direction. The multiple slits extend in a direction perpendicular to a direction in which light that has passed through the polarizing filter is polarized.
An image-pickup device according to one embodiment of the disclosure includes a spectral device and a light-receiving portion that detects light that has passed through the spectral device. The spectral device includes a polarizing filter and an optical filter. The polarizing filter transmits part of light incident on the polarizing filter, the part of light having a particular polarization component. Light that is incident on and passes through the polarizing filter is converted into linearly polarized light. Light that has passed through the polarizing filter is incident on the optical filter. The optical filter transmits light within a particular frequency range. The optical filter includes a metal layer and a dielectric layer. The dielectric layer is disposed on the metal layer. Multiple slits are formed in the metal layer. The multiple slits are arranged at equal intervals in a predetermined direction. The multiple slits extend in a direction perpendicular to a direction in which light that has passed through the polarizing filter is polarized.
Referring now to the drawings, specific embodiments of the disclosure are described below. Throughout the drawings, the same or equivalent portions are denoted with the same reference symbols and are not described repeatedly.
The image-pickup device 10 includes a spectral device 12, an outer lens 14, an inner lens 16, and a light-receiving portion 18. The spectral device 12 includes a polarizing filter 20 and an optical filter 22.
The polarizing filter 20 transmits part of light incident on the polarizing filter, the part of light having a particular polarization component (that is, light that oscillates in a particular direction). The polarizing filter 20 converts the incident light into linearly polarized light. In other words, light that is incident on and passes through the polarizing filter 20 is converted into linearly polarized light. The polarizing filter 20 is not limited to be in a particular form as long as it converts incident light into linearly polarized light. For example, a slit polarizing plate is employed as the polarizing filter 20.
The optical filter 22 is located at such a position that light that has passed through the polarizing filter 20 is incident on the optical filter 22. The optical filter 22 transmits light, most of which falls within a particular wavelength range.
The filter portions 22A and the filter portions 22B each have a rectangular shape (square shape in this embodiment) in a plan view. In the optical filter 22, the filter portions 22A and the filter portions 22B are alternately arranged in the row and column directions (X and Y directions in
Each filter portion 22A includes two metal layers 24 and one dielectric layer 26. In
One of the two metal layers 24 (referred to as a metal layer 241, below) is disposed on a support substrate, not illustrated. The support substrate includes a ground layer and a base substrate. An example of the ground layer is a silicon oxide film. The base substrate transmits light. An example of the base substrate is a glass substrate. When the image-pickup device 10 is used as an image-pickup device, a complementary metal oxide semiconductor (CMOS) device or a charge-coupled device (CCD) is used as an image-pickup element. In this case, an interlayer film formed in the process of forming a contact hole or in the process of forming a wire may be used as a ground layer. In this case, a planarizing process such as chemical-mechanical polishing (CMP) may be performed as needed.
The other one of the two metal layers 24 (hereinafter referred to as a metal layer 242) is disposed apart from the metal layer 241. The metal layer 242 is disposed apart from the metal layer 241 in a direction in which light travels.
The metal layers 24 mostly contain Al. Examples of the material of the metal layers 24 may include Ag, Au, Pt, Ti, TiN, Cu, and AlCu. The refractive index of the metal layers 24 may be within 0.35 to 4.0 in the range of visible light. In this embodiment, the refractive index of the metal layers 24 when light having a wavelength of 550 nm propagates through the metal layer 24 is 0.74.
For the sake of processing convenience, the thickness of the metal layers 24 may be within 20 to 100 nm. In this embodiment, the thickness of the metal layers 24 is 40 nm. The two metal layers 24 may have the same thickness or different thicknesses. In this embodiment, the two metal layers 24 have the same thickness.
The multiple slits 25A are formed in each of the metal layers 24. The multiple slits 25A are formed at equal intervals in a particular direction (X direction or the width direction of the metal layers 24 in the example illustrated in
A width S1 of each slit 25A is appropriately determined in accordance with an intended wavelength (selective wavelength) of light that the filter portion 22A is to transmit. The width S1 may be within 80 to 200 nm. In this embodiment, the width S1 is 100 nm. The width S1 may be within 10 to 50% of the pitch C1. In this embodiment, the width S1 is approximately 33% of the pitch C1. In the example illustrated in
The length of each slit 25A (dimension in the Y direction in
The dielectric layer 26 is disposed on the metal layers 24. Portions of the dielectric layer 26 lie in the slits 25A. Examples of the material of the dielectric layer 26 include SiN, ZnSe, SiO2, and MgF. The material of the portion of the dielectric layer 26 interposed between two metal layers 24 (the portion interposed between the two metal layers 24 in the direction in which light travels, that is, in the vertical direction in
The thickness of the dielectric layer 26 (specifically, the thickness of the portion of the dielectric layer 26 interposed between the two metal layers 24) is appropriately determined in accordance with an intended wavelength (selective wavelength) of light that the optical filter 22 is to transmit. The thickness of the dielectric layer 26 may be within 40 to 200 nm. In this embodiment, the thickness of the dielectric layer 26 is 100 nm. The thickness of the dielectric layer 26 may be within one to five times the thickness of each metal layer 24. In this embodiment, the thickness of the dielectric layer 26 is 2.5 times the thickness of each metal layer 24.
The refractive index of the dielectric layer 26 (specifically, the refractive index of the portion of the dielectric layer 26 interposed between the two metal layers 24) is appropriately determined in accordance with an intended wavelength (selective wavelength) of light that the filter portion 22A is to transmit. The refractive index of the dielectric layer 26 can be changed, for example, by changing the material of the dielectric layer 26. The refractive index of the dielectric layer 26 may be larger than 1.4 and smaller than or equal to 3.0.
Each filter portion 22A transmits part of light incident on the polarizing filter, the part of light mostly within a particular wavelength range, using a phenomenon similar to a resonance phenomenon at the interface between each metal layer 24 and the dielectric layer 26. By optimizing parameters affecting this phenomenon (such as the thickness of each metal layer 24, the width of the slits 25A in each metal layer 24, the pitch of the slits 25A, the thickness of the dielectric layer 26, or the refractive index of the dielectric layer 26), the light transmittance of the filter portion 22A can be improved.
The thickness of each metal layer 24 or the dielectric layer 26, the width S1 of the slits 25A, or the pitch C1 of the slits 25A has to be changed in accordance with the properties of the material of each layer 24 or 26 (particularly, the refractive index) or the selective wavelength. Particularly, the refractive index has to be calculated in advance for each selective wavelength through simulation since the refractive index has wavelength dependency. The selective wavelength depends on the difference L1 and the thickness of the dielectric layer 26.
The material of each layer 24 or 26 is not limited to the examples described above. Any material that causes plasmon resonance at the interface between each metal layer 24 and the dielectric layer 26 is usable. Specifically, any material having a negative dielectric constant is usable as a material of the metal layer 24. The refractive index of the dielectric layer 26 will suffice if it is higher than the refractive index (1.4) of the ground layer (silicon oxide film) on which the metal layer 241 is disposed.
Now, a method for manufacturing the optical filter 22 is described.
As illustrated in
Subsequently, as illustrated in
Referring back to
The inner lens 16 is disposed between the optical filter 22 and the light-receiving portion 18. Specifically, light that has passed through the optical filter 22 is incident on the inner lens 16. Light that has passed through the inner lens 16 is incident on the light-receiving portion 18. The inner lens 16 concentrates the incident light on the light-receiving portion 18.
The light-receiving portion 18 receives light that has passed through the inner lens 16. The light-receiving portion 18 is an image-pickup element.
As illustrated in
As illustrated in
As illustrated in
The optical filter 22 enables concurrent selection of the wavelength and the direction in which light is polarized. Here, the wavelength has a correlation with the pitch between the slits 25A or 25B. The direction in which light is polarized has a correlation with the direction in which the slits 25A or 25B extend. These parameters can be designed independently of each other.
To manufacture the optical filter 22, a single exposure mask can determine the pitch between the slits 25A or 25B or the direction in which the slits 25A or 25B extend. Thus, a single exposure process will basically suffice for manufacturing the optical filter 22 having various different filter portions (that is, selective wavelengths). Thus, the manufacturing of the optical filter 22 using a single exposure mask can significantly reduce the number of die sets or processes compared to the case of manufacturing an optical filter using an organic film or a multilayer film.
Moreover, a change of an exposure mask layout can appropriately change the selective wavelength or the direction in which light is polarized.
In addition, the optical filter can be formed by using a material usually used in a semiconductor manufacturing process such as aluminum or silicon.
The image-pickup device 10 includes the outer lens 14. Thus, the optical filter 22 has higher spectral characteristics. The reason is described below.
The optical filter 22 has low spectral characteristics (that is, low performance of transmitting light within a particular wavelength range) when light is obliquely incident on the optical filter 22. Thus, the outer lens 14 is disposed to convert light incident on the optical filter 22 into a plane wave, so that the optical filter 22 has higher spectral characteristics.
The image-pickup device 10 includes the inner lens 16. Thus, the light-receiving portion 18 has higher sensitivity to light. The reason is described below.
Light that has passed through the optical filter 22 is converted into a spherical wave. Thus, the inner lens 16 is disposed to concentrate the light that has passed through the optical filter 22 on the light-receiving portion 18, so that the light-receiving portion 18 has higher sensitivity to light.
As described above, the image-pickup device 10 includes the outer lens 14 and the inner lens 16. Thus, the image-pickup device 10 can produce an image having higher contrast.
In contrast to the case of the optical filter 22 illustrated in
For example, as illustrated in
When, for example, the polarizing filter 20 is disposed so as to be rotatable relative to the optical filter 22, the polarizing filter 20 may have multiple filter portions in each of which the direction in which slits extend or the pitch between the slits differs from the direction or the pitch in the other filter portions. In this case, the optical filter 22 may omit multiple filter portions in each of which the direction in which slits extend or the pitch between the slits differs from the direction or the pitch in the other filter portions. Thus, an exposure mask layout used for forming silts in the optical filter 22 is simplified, so that a design margin is widened.
In the first embodiment, two filter portions 22A and two filter portions 22B, that is, two pairs of filters portions having the same selective wavelength, are disposed in an area of the optical filter 22 corresponding to one pixel 18A. However, multiple filter portions disposed in the area of the optical filter 22 corresponding to one pixel 18A may individually have different selective wavelengths.
In the first embodiment, multiple filter portions disposed in the area corresponding to one pixel 18A each have slits. However, as illustrated in
In the case where multiple filter portions 22F are included, the slits 25F in all the filter portions 22F may extend in the same direction or different directions. In the case where multiple filter portions 22F are included, the slits 25F in all the filter portions 22F may be formed at the same pitch or different pitches.
The opening 25G in the filter portion 22G may have any shape. For example, the opening 25G may be square, as illustrated in
When the filter portion 22G has light transmittance excessively higher than the light transmittance of the filter portion 22F, the light transmittance of the filter portion 22G can be changed to intended light transmittance by adjusting the area of the opening 25G. In the example illustrated in
The form illustrated in
The method for adjusting the light transmittance of the filter portion 22G and the light transmittance of the filter portion 22F is not limited to the above-described adjustment of the area of the opening 25G. For example, besides the adjustment of the area of the opening 25G, the length of the slits 25F may also be adjusted as needed. In some cases, only the adjustment of the length of the slits 25F may suffice.
In the case where multiple filter portions 22G are included, the openings 25G in the filter portions 22G may have the same size or different sizes.
For example, in the first embodiment, the image-pickup device 10 may omit the outer lens 14 and the inner lens 16.
Referring to
Simulations in both cases were performed by finite difference time domain (FDTD). The reason why the peak wavelength differs between
As illustrated in
In contrast to the optical filter 22, the optical filter 222 does not include the metal layer 242. Thus, the shape of the metal layer 241 is more easily fixed when slits are formed therein. Thus, the optical filter 222 is manufactured at higher yield than in the case of the optical filter 22.
Referring to
In contrast to the image-pickup device 10, the image-pickup device 10B includes a light source 32. The light from the light source 32 passes through the polarizing filter 20. The light that has passed through the polarizing filter 20 is shone on an object 34 and reflected off the object 34. The light reflected off the object 34 is incident on the optical filter 22. The light incident on the optical filter 22 is then incident on the light-receiving portion 18. Thus, the spectrum of the object 34 is obtained.
When the surface of the object 34 is to be observed, the direction in which slits in the optical filter 22 extend may be rendered perpendicular to the direction in which slits in the polarizing filter 20 extend.
To obtain information inside the object 34, the direction in which the slits in the polarizing filter 20 extend and the direction in which the slits in the optical filter 22 extend are adjusted in consideration of an optical path difference. The information inside the object 34 is a diffuse reflection component. A polarized mirror reflection component (for example, S-wave or P-wave) functions as a noise for a diffuse reflection component. Thus, this noise is reduced by adjusting the direction in which the slits in the polarizing filter 20 extend and the direction in which the slits in the optical filter 22 extend. Specifically, the direction in which the slits in the polarizing filter 20 extend is rendered perpendicular to the direction in which the slits in the optical filter 22 extend.
The polarizing filter 20 may be installed on the light source 32 or on the object 34.
Thus far, embodiments of the disclosure have been described in detail. These embodiments, however, are mere examples and the disclosure is not at all limited by the above-described embodiments.
The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2015-228116 filed in the Japan Patent Office on Nov. 20, 2015, the entire contents of which are hereby incorporated by reference.
It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.
Number | Date | Country | Kind |
---|---|---|---|
2015-228116 | Nov 2015 | JP | national |