Patent Application
20120176676
1. Field of the Invention
The present invention relates to a method of producing a wire-grid polarizer utilizing a line-and-space structure of a metal and relates to a wire-grid polarizer.
2. Description of the Related Art
In wire-grid polarizers, a narrow pitch structure has been realized by progress in manufacturing technology. The pitch of a wire-grid polarizer should be smaller than a half of the wavelength to be used in order to prevent occurrence of diffraction of light at the wavelength. Accordingly, wire-grid polarizers that are usually used at the visible range are required to have a pitch of not greater than 200 nm.
Currently, commercially available wire-grid polarizers are each composed of an aluminum line portion and an air space portion on a substrate and have a structure with a pitch of not greater than 150 nm. Such a narrow pitch structure is produced by, for example, photolithography, dry etching, or vacuum deposition. Apparatuses that are used in these methods are expensive, and thereby manufacturing costs of devices may increase. For example, Japanese Patent Laid-Open No. 2004-77831 discloses a structure and a method of producing the structure, as countermeasures against this increase in cost.
The method disclosed in Japanese Patent Laid-Open No. 2004-77831 uses a damascene process where line-like grooves are formed on a substrate, the grooves are filled with a metal, and the metal protruding from the grooves is removed. In this method, the number of steps is reduced by employing the damascene process, and a high aspect ratio structure of a metal is obtained by the embedding structure. Furthermore, an inexpensive wire-grid polarizer that is excellent in extinction performance and low in device insertion loss is provided by the structure with a high aspect ratio.
However, even in the method using the damascene process, photolithography or dry etching must be performed for forming grooves on a substrate. In addition, vacuum deposition must be performed for filling the grooves with a metal. Consequently, the produced devices become expensive. Furthermore, the wire-grid polarizer having the embedding structure has a problem of that the extinction ratio is lower than that in the case of a structure having an air space portion when compared at the same aspect ratios of the wire portions.
The present invention provides a method of producing a wire-grid polarizer that is used at the visible range and has a high extinction ratio inexpensively by not using expensive manufacturing apparatuses, and provides a wire-grid polarizer.
The method of producing a wire-grid polarizer of the present invention includes forming a line-and-space structure having a line portion and a space portion on a substrate, forming an embedded metal portion by filling the space portion of the line-and-space structure with a molten metal and solidifying the metal, and forming a protective layer on the line portion of the line-and-space structure and the embedded metal portion. The metal has a melting point of not higher than 650° C., and the solidified metal has an average extinction coefficient of higher than 5.0 at a wavelength range of 400 to 700 nm.
The wire-grid polarizer can be produced without using expensive process and apparatus, and the produced embedded-type wire-grid polarizer has high extinction ratio and transmittance and is excellent in optical characteristics.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
In the method of producing a wire-grid polarizer according to an embodiment of the present invention, a fine line-and-space structure is formed on a transparent substrate, the space portion of the structure is filled with a molten metal, and a protective film is formed thereon after solidification of the metal. The metal filling the space portion is a metal material having a melting point of not higher than 650° C., and the metal material, after the solidification, has an average extinction coefficient of higher than 5.0 or has an average extinction coefficient of higher than 4.5 and an average refractive index of less than 1, at a wavelength range of 400 to 700 nm. The optical constant of the metal is represented by n−ik, wherein n denotes the refractive index, and k denotes the extinction coefficient. The light incident plane is defined as the face including the normal line perpendicular to the wire grid, the S-polarized light is defined as light of vibrational component parallel to the wire grid, and the P-polarized light is defined as light of vibrational component perpendicular to the wire grid. The behaviors of the S-polarized light and the P-polarized light are switched with each other by turning the wire-grid polarizer by 90°.
Since the melting point of the metal is not higher than 650° C., the space portion can be directly filled with the molten metal without damaging the substrate and the line-and-space structure having a fine scale less than the wavelength. Thus, an inexpensive manufacturing method, such as a reflow method, can be employed. Consequently, a wire-grid polarizer can be produced at a low cost without performing expensive vacuum deposition, photolithography, and dry etching.
In addition, when the metal filling the space portion has an average extinction coefficient of higher than 5.0 or an average extinction coefficient of higher than 4.5 and an average refractive index of less than 1 at the visible range, a wire-grid polarizer having high extinction ratio and transmittance can be realized. Thus, a metal material that satisfies both such melting point and optical constant is selected as the metal.
The metal remaining on the line portion of the line-and-space structure is removed so that the upper surface of the line portion is exposed. If the metal remains on the line portion, it is difficult to realize the desired extinction ratio and transmittance. The metal can be removed by a known method such as lapping or polishing.
Specifically, the metal material is pure-In or an alloy of at least two of Al, Mg, In, Sn, Zn, Ag, and Ge. Al, Ag, and Mg each have a melting point higher than 650° C., but each have a low refractive index (n) and a high extinction coefficient (k) at the visible range. In the production of a wire-grid polarizer, a higher value of k is ideal for obtaining advantageous characteristics. In the case of a k of higher than 4.5 and not higher than 5.0, a lower value of n provides advantageous characteristics. Contrarily, In, Sn, and Zn have low melting points.
The required melting point and the required optical constant can be simultaneously realized by an alloy formed from these metals. It is known that though the melting points of Al and Mg as simple metals are higher than 650° C., the melting point of a eutectic alloy of Al and Mg formed in such a manner that the amount of Mg is about 37.4% by weight based on that of Al is lowered to about 451.5° C. Thus, a eutectic composition can decrease the melting point and can also adjust the optical constant within a desired range. Similarly, also in Al and Ge, a eutectic composition can decrease the melting point such that the melting point of an alloy of Al and Ge in a composition ratio of 90:10 (% by atom) is lowered to about 590° C. In this alloy, a high optical constant, i.e., a high average extinction coefficient k of 5.05 can be realized at the visible range.
Furthermore, in alloys containing Mg, In, and Sn, the surface tensions of molten alloys are low. Accordingly, the fine space portion can be easily filled with an alloy formed from these metals. In particular, Mg shows high effects. Incidentally, pure-In is the only pure metal that can be applied to the method of the present invention.
The refractive index n of the line portion of the line-and-space structure and the refractive index ns of the substrate satisfy a relationship: 1<n<ns. It is ideal that a polarizer produced by filling the space portion of the line-and-space structure with the above-mentioned metal have a refractive index of 1. However, a refractive index of 1 is realized only in a vacuum or a gas and cannot be realized by the space filled with a metal. A wire-grid polarizer having a high extinction ratio can be also produced by lowering the refractive index of the line portion. In the case where the refractive index n of the line portion is higher than the refractive index ns of the substrate, the extinction ratio is decreased due to a decrease in transmittance of P-polarized light and an increase in leakage of S-polarized light.
The line-and-space structure with a fine scale less than the wavelength is formed by nano-imprinting a sol-gel material including a siloxane as a main component. In gelation of the sol-gel material, in general, a certain type of solvent functions as an activator to accelerate formation of microparticles of several nanometers to several tens nanometers. When the sol-gel material is hardened in such a structure of the microparticles, a porous structure is formed, and a fine-hollow structure is maintained. As a result, a line structure having a low refractive index can be produced. The curing temperature for hardening the material while maintaining the porous structure is 150° C. or higher, preferably 200° C. or higher, and the upper limit thereof is 650° C. A temperature higher than this range may cause problems, such as an increase in refractive index or occurrence of absorption of light at the wavelength used. Hardening the sol-gel material under conditions of a temperature of less than 150° C. causes various disadvantages, such as a reduction in hardness, an increase in refractive index, and low solvent resistance of the resulting coating. This temperature varies depending on the sol-gel material and therefore needs to be determined according to the material to be used. The lower limit of the temperature must be lower than the melting point of the embedded metal.
A line-and-space structure with a fine scale less than the wavelength can be produced by utilizing a sol-gel material and nano-imprinting without performing vacuum deposition, photolithography, and dry etching.
The protective layer is heat cured after application of the sol-gel material including siloxane as the main component. By doing so, a low refractive index is achieved by the same mechanism as in the line portion having a low refractive index. By using such a sol-gel material, a wire-grid polarizer can be produced without performing vacuum deposition.
As an ideal optical constant of a metal in the wire-grid polarizer, the metal material has a higher extinction coefficient k. In the case where the average value of the extinction coefficient k (average extinction coefficient) at the visible range is higher than 5.0, a wire-grid polarizer having a high P-polarized light transmittance and a high extinction ratio can be produced, even if the refractive index n is any level that is of a general metal. Preferred examples of such a material include the following binary system alloys: Al—Mg alloys where the content ratio of Al to Mg is 0<Al (% by atom)≦94.5, Al—In alloys where the content ratio of Al to In is 5.9≦Al (% by atom)≦12.5, Al—In alloys where the content ratio of Al to In is 94.2≦Al (% by atom)≦97.5, Al—Zn alloys where the content ratio of Al to Zn is 0<Al (% by atom)≦96.9, Al—Ag alloys where the content ratio of Al to Ag is 50.7≦Al (% by atom)≦93.0, Mg—Zn alloys where the content ratio of Mg to Zn is 74.3≦Mg (% by atom)≦98.0, and Al—Sn alloys where the content ratio of Al to Sn is 7.1≦Al (% by atom)≦96.0. In alloys of ternary or more systems, it is difficult to unambiguously define the composition, but a polarizer having a high extinction ratio to exhibit a high efficiency can be obtained by satisfying the melting point and refractive index conditions according to the present invention.
Contrarily, in the case where the average value of the extinction coefficient k at the visible range is not higher than 4.5, the S-polarized light transmittance increases, resulting in a difficulty in improving the extinction ratio. In the case where the extinction coefficient k is higher than 4.5 and not higher than 5.0, a wire-grid polarizer having a high P-polarized light transmittance and a high extinction ratio can be produced only when the average value of the refractive index n (average refractive index) at the visible range is less than 1. Preferred examples of such a material include the following binary system alloys: Al—Mg alloys where the content ratio of Al to Mg is 0<Al (% by atom)≦94.5, Al—In alloys where the content ratio of Al to In is 0≦Al (% by atom)≦12.5, Al—Zn alloys where the content ratio of Al to Zn is 0<Al (% by atom)≦96.9, Al—Ag alloys where the content ratio of Al to Ag is 35.7≦Al (% by atom)≦93.0, Al—Ge alloys where the content ratio of Al to Ge is 66.8≦Al (% by atom)≦97.0, and Mg—Zn alloys where the content ratio of Mg to Zn is 0≦Mg (% by atom)≦98.0. In particular, in the case where the wire-grid polarizer is produced using an alloy having such optical constants, the reflectance of the S-polarized light increases. Accordingly, the wire-grid polarizer can be also used as a polarization beam splitter that utilizes reflected light. Incidentally, a metal material showing a highest average value of k at the visible range is Al, and the value is 6.65. Consequently, it is difficult that these alloy systems have an extinction coefficient k higher than this value.
As the metal for a wire-grid polarizer that functions at the visible range, Al can realize the highest P-polarized light transmittance and extinction ratio. However, Al has a high melting point of 660° C. and, therefore, cannot be applied to the configuration of the present invention. Accordingly, Al is used as an alloy having a metal composition that reduces the melting point while maintaining the optical constants within feasible ranges, compared with Al.
The pitch P and the space width S of the line-and-space structure satisfy the requirements: 50 nm≦P≦200 nm and 0.2≦S/P≦0.4. In these ranges, a wire-grid polarizer having suitable polarization characteristics is provided. The upper limit value of the pitch is a maximum pitch that does not cause diffraction in a wire-grid polarizer using vertical incident visible light. Diffraction does not occur in the range of pitch not greater than this pitch, and therefore the wire-grid polarizer can be satisfactorily used. In the case where the space for being filled with a molten metal is narrow, it is difficult to fill the space with a metal. Herein, a minimum value of the space width of 10 nm is the lower limit. In the case of a value of S/P exceeding 0.4, the transmittance of P-polarized light highly decreases, which is undesirable for a wire-grid polarizer. Even if the pitch and the space width are outside these ranges, a wire-grid polarizer having satisfactory P-polarized light transmittance and extinction ratio can be realized when the metal filling the space has certain optical constants. Therefore, the pitch and the space width are not limited to the above-mentioned ranges.
The depth D1 of the space portion of the line-and-space structure satisfies the requirement: 90 nm≦D1≦250 nm. Such a high aspect structure can be realized by filling the space having such a depth with a metal, and thereby a wire-grid polarizer having a high extinction ratio is provided. In the case of a depth of shallower than this, though the transmittance of P-polarized light is high, the amount of leakage of S-polarized light is large. Contrarily, in the case of a depth of deeper than this, the amount of leakage of S-polarized light is small, but the transmittance of P-polarized light is low.
The above-described alloys are effective as a metal that can satisfactorily fill the narrow and deep space having a high aspect ratio and can realize a high extinction ratio while maintaining a high transmittance of P-polarized light. Even if the depth of the space portion is outside the above-mentioned range, a wire-grid polarizer having satisfactory P-polarized light transmittance and extinction ratio can be realized when the metal filling the space has certain optical constants. Therefore, the depth is not limited to the above-mentioned range.
The refractive index n2 and the thickness D2 of the protective layer and the central wavelength λ of the light used satisfy the requirements: 1.2≦n2≦1.4 and 0.2λ≦n2˜D2≦0.3λ. In this range, a reflection-preventing effect that is effective at the visible light wavelength range is exhibited to provide a wire-grid polarizer having a high transmittance. In a wire-grid polarizer made of a metal, since the optical constant of the metal approaches an ideal state at the longer wavelength side, the transmittance of P-polarized light and also the extinction ratio are high at the longer wavelength side. Against this, within the above-mentioned ranges of the thickness and the refractive index of the protective film, it is possible to adjust the transmittance of P-polarized light to the maximum at the central wavelength of the visible light range being used. The protective layer also functions as an antioxidant film for the metal filling the space, where the metal is an alloy system that tends to be oxidized.
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Similarly, a coating of the same alloy filling the space portion was formed on a quartz substrate by a reflow method, and the optical constants thereof were measured from the back surface with a commercially available meter (ellipsometer available from J. A. Woollam Co.). The refractive index n and the extinction coefficient k at a wavelength range of 400 to 700 nm were 0.335≦n≦1.194 (average refractive index: 0.700) and 3.84≦k≦6.58 (average extinction coefficient: 5.28), respectively. The average value of k was higher than 5, which was within the condition range of the present invention.
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Thus, a high-quality and inexpensive wire-grid polarizer was able to be produced without performing, for example, expensive photolithography, dry etching, and vacuum deposition.
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Example are shown in
In this example, a quartz substrate was used as the substrate 1 in Example 1. The nickel mold 3 used in the step in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Example are shown in
In this Example, a white plate was used as the substrate 1 in Example 3. In the step shown in
The spectral transmittance characteristics of the wire-grid polarizer prepared in this Example are shown in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Example are shown in
In this Example, a base material (S-BSL7) was used as the substrate 1 in Example 4. In the step shown in
The spectral transmittance characteristics of the wire-grid polarizer prepared in this Example are shown in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Example are shown in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Comparative Example are shown in
The same method as in Example 1 was conducted using a nickel mold 3 having a line width of 31 nm, a space width of 109 nm, and a space depth of 222 nm in the step shown in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Comparative Example are shown in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Comparative Example are shown in
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The spectral transmittance characteristics of the wire-grid polarizer prepared in this Comparative Example are shown in
The wire-grid polarizer of the present invention can be satisfactorily used as a polarizer of, for example, a liquid crystal projector or a liquid crystal display and also can be used as a polarization beam splitter for separating incident light into different linear polarized light components.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2011-001017 filed Jan. 6, 2011, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-001017 | Jan 2011 | JP | national |
