Abstract:
Optical multilayer thin-film filters (OMTFFs) are disclosed. An exemplary filter includes a transparent substrate, a multilayer film (MF) on a surface of the substrate, and a top layer. The MF is of alternatingly laminated layers of a high-refractive-index (HRI) material and a low-refractive-index (LRI) material. The top layer is on an uppermost layer of the MF and is of a material having atoms of lower atomic weight than atoms of either the HRI or LRI materials. The OMTFFs are made in a vacuum environment by alternatingly laminating respective thin films of the HRI and LRI materials on the substrate. The top layer is formed on the MF. Between forming the MF and top layer is a suppression step in which the newly formed MF is exposed to moisture by briefly venting the vacuum to atmosphere. The moisture inhibits migration of the low-molecular-weight atoms into the HRI and LRI materials.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/330,421, filed Dec. 8, 2008, which claims priority to, and the benefit of, Japan Patent Application No. 2007-324227, filed on Dec. 17, 2007, in the Japan Patent Office, both of which are incorporated herein by reference in their respective entireties. 
     
    
     FIELD 
       [0002]    This disclosure pertains to, inter alia, multilayer-film optical filters that include a transmissive, surficial metal film to reduce or eliminate accumulation of charges on the filter. Also disclosed are methods for making such multilayer-film optical filters, including formation of the metal film on a surface of an OLPF (optical low-pass filter) comprising a substrate such as quartz crystal. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Conventional digital imaging devices such as digital still cameras and digital video cameras include a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) solid-state image sensor. Placed on or near the image-receiving surface of the image sensor is an optical low-pass filter (OLPF), comprising a glass or quartz crystal substrate. The OLPF passes lower-frequency light components and blocks (e.g., by reflection) higher-frequency light components. Using an OLPF, fine patterns mainly having large luminance differences can be defocused. For example, the solid-state image sensor tends to generate an interference pattern (moiré pattern) whenever the image includes ordered fine patterns. The sensor also tends to generate color moiré patterns that add unwanted color to the detected image when the photographic subject has large luminance differences such as hair illuminated by backlight. The OLPF removes these interference patterns (moiré patterns or color moiré patterns) by defocusing the image and blurring the edges of the image. 
         [0004]    Placed on or near the front or rear surface of a conventional OLPF is an optical multilayer thin-film filter (OMTFF). The OMTFF passes only optical wavelengths in a range sensed by the human eye and blocks transmission of infrared radiation. Thus, the wavelength-detection range of the solid-state image sensor, which has high sensitivity to infrared radiation, is brought closer to actual human vision. Because the substrate of the OLPF is made of glass or quartz crystal, it tends to become charged by its own piezoelectric effect. If the OLPF becomes electrically charged, the OMTFF also acquires a surface charge, which causes the OMTFF to attract and adsorb dust particles. Hence, especially whenever interchangeable lenses of a digital single-lens reflex camera are changed, for example, the filter is exposed to outside air and adsorbs dust from the air. The adsorbed dust also tends to be charged, making it difficult to remove the dust from the filter. 
         [0005]    Japan Unexamined Patent Publication No. 2007-298951 discusses a method for removing static charge from an OMTFF by depositing a surficial layer of ITO (indium tin oxide) on the filter to provide the filter surface with a charge-conducting material. ITO transmits 90% of incident optical wavelengths, so the method does not change the optical characteristics of the filter in any significant way. 
         [0006]    However, strictly speaking, the method of depositing ITO tends to contaminate the interior of the deposited multilayer-film layers. The contamination tends to degrade the optical performance of the OMTFF. Contamination arises because ITO is deposited on the surface of the multilayer film after forming the multilayer film. The multilayer film is formed by alternatingly depositing layers of a first dielectric film having high refractive index and layers of a second dielectric film having low refractive index. Also, indium (In) is a rare metal; since ITO is used for liquid-crystal panels and flat-panel displays (FPDs) such as organic EL, its anticipated price escalation and deficiency of supply in the future is a concern. 
         [0007]    The current invention prevents accumulation of static charges on the OLPF by providing the associated OMTFF with a charged-conductive surface, but without having to use ITO. 
       SUMMARY 
       [0008]    According to a first aspect of the invention, optical multilayer thin-film filters (OMTFFs) having particular configurations are provided. An embodiment of the OMTFF comprises a transparent substrate and a highly refractive multilayer film comprising alternatingly laminated thin layers of a high-refractive-index material and of a low-refractive-index material on a surface of the substrate. Formed on the multilayer film is a layer of a charge-conductive material having lower atomic weight than the high-refractive-index and low-refractive-index materials. An OMTFF having such a configuration provides good charge dissipation without having to use ITO. Since the charge-conductive layer readily conducts static charges away from the optical multilayer film, adsorption of dust onto the layer is at least substantially inhibited. 
         [0009]    The OMTFF can be used in combination with an infrared-blocking filter that transmits visible light and reflects or otherwise blocks transmission of infrared light. The OMTFF also confers an antistatic property to the infrared-blocking filter; when the filters are used in conjunction with an image-forming and recording device such as a camera, dust does not adversely affect performance of the image-forming and recording device. 
         [0010]    The high- and low-refractive-index thin films desirably are layers of respective oxides. The “final” or uppermost layer, formed atop the multilayer film, is not an oxide. The final layer is formed in a manner resulting in formation of a metal film. Some of the material of the final layer penetrates into the upper high- and low-refractive-index thin films, thereby providing good bonding of the metal film to the multilayer film and good removal of charges from the filter. Some of the material of the final layer reacts with oxygen atoms in the oxides of the high- and low-refractive-index films. Thus, some of the deposited material of the final layer becomes oxides of the metal of which the final layer is formed. This further facilitates the antistatic property of the filter. 
         [0011]    The high-refractive-index layers desirably comprise any of TiO 2 , Nb 2 O 5 , and Ta 2 O 5 , or of compound oxides based on one or more of TiO 2 , Nb 2 O 5 , and Ta 2 O 5 . The low-refractive-index layers desirably comprise either SiO 2  or Bi 2 O 3 , or of compound oxides based on one or more of La 2 O 3  and Al 2 O 3 . 
         [0012]    In many embodiments the final layer is formed from MgF 2  or LiF. MgF 2  and LiF have low molecular weights so that they can diffuse into the material of the high- and low-refractive-index films, particularly the upper films in the multilayer structure. 
         [0013]    In many embodiments the transparent substrate is glass or quartz crystal. Both materials are suitable for use in making an optical filter. 
         [0014]    According to another aspect of the invention, methods are provided for manufacturing an OMTFF. An embodiment of the method includes a first deposition step, in which alternating thin layers of a high-refractive-index material and of a low-refractive-index material are deposited superposedly on a transparent substrate. Thus, in the first deposition step, a multilayer film is formed. In a second deposition step a “top” layer is formed, atop the multilayer film, of a material having a lower molecular weight than either the high- or low-refractive-index materials. In a suppression step the amount of lower-molecular-weight material allowed to diffuse into the high- and low-refractive-index materials is controlled. By appropriate control being exercised in the suppression step, the deposited low-molecular-weight material is substantially prevented from entering the high- and low-refractive-index materials easily, allowing a practical OMTFF to be manufactured. 
         [0015]    The suppression step can include, for example, establishing conditions in which the high- and low-refractive-index materials can absorb moisture. Moisture absorption by the high- and low-refractive-index materials largely prevents the low-molecular-weight material of the top layer from entering the high- and low-refractive-index materials. 
         [0016]    Desirably, in the first deposition step, the high-refractive-index layers and low-refractive-index layers of the multilayer film are formed alternatingly by vacuum deposition in a vacuum chamber. Then, in a suppression step after the first deposition step, the vacuum chamber is vented to atmosphere. In the subsequent second deposition step, the vacuum chamber is returned to a vacuum state before commencing deposition of the low-molecular-weight material to form the top layer. By venting the chamber to atmosphere during the suppression step, moisture contained in the air is absorbed in the layers of high- and low-refractive-index materials. With this simple venting step an OMTFF is fabricated that has high conductivity. 
         [0017]    Various embodiments of an OMTFF include a metal film formed without having to use a transparent electrode material such as ITO in the top (also called “final”) layer. Thus, the surface of the OMTFF effectively eliminates static from the OLPF with which the OMTFF is often used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a perspective view of an ion-assisted deposition apparatus used for fabricating various embodiments of optical multilayer thin-film filters. 
           [0019]      FIG. 2  is a schematic elevational view of a quartz-crystal substrate  20  on which multiple thin layers of high-refractive-index material and low-refractive-index material are laminated in an alternating manner to form a multilayer film. 
           [0020]      FIG. 3  is a graph of intensity versus depth exhibited by the upper layers of an embodiment of an optical multilayer thin-film filter. 
           [0021]      FIG. 4  is a graph of intensity versus depth exhibited by the upper layers of a defective optical multilayer thin-film filter. 
           [0022]      FIG. 5  is an optical diagram of an embodiment of an imaging module of a digital still camera including an optical multilayer thin-film filter. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Referring first to  FIG. 2 , an embodiment of an optical multilayer thin-film filter (OMTFF)  30  exhibiting a charge-eliminating function is formed by laminating, in an alternating manner, respective thin films of: (a) an oxide having a high refractive index and (b) an oxide having low refractive index. The multilayer film is formed from multiple superposed layer pairs of these materials. The resulting multilayer film has a top (uppermost) surface on which a layer of a non-oxide having low refractive index is formed. For example, titanium dioxide  32  (TiO 2 ) can be used as the high-refractive-index material, and silicon dioxide  33  (SiO 2 ) can be used as the low-refractive-index material. For effective blocking of incident infrared radiation, the multilayer film includes 20 to 60 layer pairs of the high-refractive-index material  32  and low-refractive-index material  33 . A “final” (top) layer of magnesium fluoride  31  (MgF 2 ) is formed on the multilayer film. The top layer  31  removes and prevents charge accumulations on the filter  30 . 
         [0024]    Incident light on the OMTFF undergoes multiple reflections between the surfaces of the layers of the multilayer film. Constructive interference of multiply reflected wavefronts produces a transmission maximum for the desired wavelength(s), resulting in a transmission-pass band. Destructive interference reduces transmission of undesired wavelength(s) to nearly zero, resulting in a reflection band(s). Therefore, it will be appreciated that the layers of the multilayer film must be made while exercising tight control of the index of refraction and physical thickness of each layer. 
       Method for Manufacturing Optical Multilayer Thin-Film Filter 
       [0025]    The optical multilayer thin-film filter (OMTFF)  30  desirably has a stable thin-film structure having optical characteristics that are unaffected by changes of temperature or humidity. To such end, the OMTFF  30  desirably is formed using IAD (Ion-Assisted Deposition). A general configuration of an apparatus  10  for performing IAD is shown in  FIG. 1 . In the IAD apparatus  10 , deposition material (from which a layer is to be formed on a substrate  20 ) is evaporated in a vacuum chamber  19  and directed toward the substrate  20  on which the evaporated material becomes deposited. 
         [0026]    The IAD apparatus  10  includes an ion source  13  located inside the vacuum chamber  19 . During operation the chamber  19  is evacuated to an appropriate vacuum level. The ion source  13  is an important feature for performing “ion-assisted” deposition. Ions  22  from the ion source  13  impart kinetic energy to particles of vacuum-evaporated material produced in the chamber  19 . The ions  22  also facilitate formation of amorphous layers on the substrate  20  rather than crystalline layers. Forming amorphous layers enhances the density of the layers so that the layers exhibit less wavelength shift in air. 
         [0027]    In the IAD apparatus  10  the ions  22  (being positively charged) could cause charge accumulation in and on the thin layers being formed on the substrate  20 . To prevent charge accumulation, electrons  23  (having negative charge) are also irradiated in the chamber  19  from a “charge-neutralizing device”  14 . 
         [0028]    For supplying material for deposition of the respective layers of the multilayer film, the IAD apparatus  10  includes a first evaporation source  11  and a second evaporation source  12 . When bombarded by electrons from a respective electron source  17 , the first evaporation source  11  releases, in the vacuum environment in the chamber  19 , vaporized material having high refractive index. Similarly, when bombarded by electrons from a respective electron source  17 , the second evaporation source  12  releases, in the vacuum environment in the chamber  19 , vaporized material having low refractive index. Items  22  in the figure represent vaporized material. A respective shutter  16  is arranged above each evaporation source  11 ,  12  so that a selected one of the sources  11 ,  12  is releasing vaporized material at any given time. A substrate dome  15  is arranged above the shutters  16 . One or more substrates  20  (made of quartz crystal or other suitable substrate material) are mounted inside the substrate dome  15 . By rotating the base dome  15  during production of vaporized layer-forming material in the chamber  19 , the particles  22  of vaporized material are deposited evenly on the crystal substrate  20 . 
         [0029]    A first deposition step is directed to forming the alternating layers of the multi-layer film on a surface of each of the substrates  20 . The IAD apparatus  10  irradiates electrons from the respective electron source (electron “gun”)  17  to the first evaporation source  11 , and radiates electrons from the respective electron source  17  to the second evaporation source  12 . From the electron bombardments, the respective materials of each evaporating source  11 ,  12  are evaporated in the chamber  19 . Releasing evaporated material from either source  11 ,  12  is controlled by opening or closing the respective shutters  16  for respective designated times. To form alternating layers on the substrates  20 , the shutters  16  are opened and closed in an alternating manner for respective amounts of time. Thus, layers of the material of the first evaporation source  11  and layers of the material of the second evaporation source  12  are deposited alternatingly with designated respective thicknesses on the substrates  20 . For example, the first evaporation source  11  is titanium for forming titanium dioxide layers  32 , and the second evaporation source  12  is silicon for forming silicon dioxide layers  33 . The corresponding oxides are formed in the chamber  19  in the presence of oxygen gas  21  introduced into the chamber. 
         [0030]    A second deposition step is performed after completion of the multilayer film in the first deposition step. For the second deposition step a third evaporation source (not shown) is arranged in the chamber  19 . In one embodiment the third evaporation source contains magnesium fluoride  31 , which is evaporated by irradiating electrons on it from a respective electric gun  17 . Thus, a top film is formed on the multilayer film on the substrates  20 . 
         [0031]    In the first deposition step, during formation of the multilayer film, the vacuum chamber  19  of the IAD apparatus maintains the required vacuum level(s) as alternating layers of titanium dioxide  32  and silicon dioxide  33  are formed. After completing the first deposition step, but before commencing the second deposition step, a “suppression step” can be performed. During the suppression step the vacuum chamber  19  of the IAD apparatus is vented to atmosphere. Then the vacuum chamber is re-evacuated before commencing the second deposition step. The vacuum level is appropriate for forming the “top” layer of magnesium fluoride  31 . By momentarily venting the vacuum chamber  19  to atmosphere, moisture (in the form of water vapor) in the air enters the vacuum chamber  19 . Some of this moisture is absorbed by the substrates  20  attached to the substrate dome  15 . More specifically, venting the vacuum chamber  19  to atmosphere after the first deposition step allows moisture to enter the chamber and contact the layers of titanium dioxide and silica dioxide formed during the first deposition step. The moisture also limits the amount of magnesium (Mg), in the second deposition step, allowed to penetrate into the layers of titanium dioxide and silicon dioxide. 
       Configuration of the Optical Multilayer-Film Filter 
       [0032]    The light-transmittance of an OMTFF  30  is similar to the transmittance of a transparent thin film. The transmittance of a thin film is defined as the product of the index of refraction and the physical thickness of the thin film. Hence, the index of refraction, thickness, and number of laminations are selected and controlled to achieve a desired reflectivity for incident infrared radiation. Generally, the thickness is established as 1/4 the wavelength of incident light to maximize refraction or transmittance. 
         [0033]      FIG. 2  schematically depicts multiple thin films laminated on a crystal substrate  20  by the IAD apparatus  10 . To reflect incident infrared radiation, each titanium dioxide layer  32  (a transparent thin film having a high index of refraction) is deposited as a first layer, and each silicon dioxide layer  33  (a transparent thin film having a low index of refraction) is deposited as a second layer. The layers are laminated alternatingly (1, 2, 3, 4, . . . , n−3, n−2, n−1) so that multiple layer pairs are formed. Each layer pair comprises a respective first layer and a respective second layer. As a “final” layer (layer n), a layer of magnesium fluoride  31 , having a low index of refraction, is laminated on the n−1 layer. The n−1 layer is a layer of silicon dioxide  33  having low index of refraction. Thus, the n layer and n−1 layer are both made of materials having low respective indices of refraction, and collectively form a thin film comprising two different materials. The number of layers of silicon dioxide  33  and of titanium dioxide  32  is about 20 to 60 layers. 
         [0034]    The magnesium (Mg) in the “final” magnesium fluoride layer (layer n)  31  has a smaller atomic radius than either the low-refractive-index material or the high-refractive-index material. The atomic weight of Mg is 24.305, of fluorine (F) is 18.998, of silicon (Si) is 28.085, and of titanium (Ti) is 47.867. Also, Mg combines with oxygen easily. Formation of the final layer  31  includes some diffusion of Mg into the underlying silicon dioxide layer  33 . But, the amount of Mg actually penetrating and the depth of penetration desirably are controlled. As Mg enters the silicon dioxide layer  33 , it forms magnesium oxide (MgO) by combining with oxygen in the layer  33 . Some Mg also enters the titanium dioxide layer  32  beneath the uppermost silicon dioxide layer  33 , and forms titanium monoxide (TiO) by combining with oxygen in the layer. 
         [0035]    Both magnesium oxide and titanium monoxide have electrical conductivity in the manner of metal films. By forming the magnesium fluoride layer  31  as the “final” layer, a metal top film is essentially formed on the top surface of the OMTFF  30  that can eliminate charge accumulation. 
         [0036]      FIG. 3  is a graph showing that, in the OMTFF  30 , some magnesium (Mg) from layer n has entered the top titanium dioxide layer n−1. The X-axis is depth from the surface of the OMTFF  30 , and the Y-axis is intensity of each element by “counts” (essentially a measure of concentration). Note that fluorine (F) and oxygen ( 0 ) are not shown in order to make the graph comprehensible. 
         [0037]    As understood from the graph, magnesium (Mg) in layer n shows a peak near a depth of  50  nm from the surface of the OMTFF  30 . The intensity of Mg declines substantially by the beginning of layer n−2, but some Mg is also observed to enter the titanium dioxide layer  32  (n−2). In the titanium dioxide layer n−2, the Mg is in the range of 10 to 50 counts, as shown in the broken-line frame A, indicating formation of magnesium oxide and titanium monoxide in this layer. Mg becomes less than or equal to 10 counts below layer n−3, indicating that the Mg in this layer may have come from residual Mg in the vacuum chamber  19 . 
         [0038]    The silicon (Si) of the silicon dioxide of layer n−1 shows a peak around 70 nm from the surface of the OMTFF  30 . In layer n−2, the Si count is two orders of magnitude lower. The titanium (Ti) from titanium dioxide in layer n−2 is three orders of magnitude lower in layer n−3. Thus, in each successive layer the main element alternates. 
         [0039]    From these results, in the IAD apparatus  10 , each material is deposited on the crystal substrate  20  to form a multilayer structure in which each layer has a designated respective thickness. Due to its particular characteristics, Mg enters moderately into the first two underlying layers. As a result, the OMTFF  30  removes incident infrared radiation and also has a top metal film to eliminate charge accumulation. 
         [0040]      FIG. 4  is a graph, similar to  FIG. 3 , of a defective OMTFF  30  in which excessive Mg has entered the underlying titanium dioxide layer n−2. As indicated in the broken-line frame B, 100 to 200 counts of Mg entered the titanium dioxide (n−2) layer. 
         [0041]    In forming the defective OMTFF  30  profiled in  FIG. 4 , titanium dioxide  32  and silicon dioxide  33  were laminated alternatingly in the first deposition step. The magnesium fluoride  31  layer was laminated in the second deposition step while vacuum conditions in the chamber  19  were maintained (i.e., without an intervening suppression step in which the chamber was temporarily vented to atmosphere). If the second deposition step is performed without any disruption in the vacuum condition, the Mg tends to penetrate more into the titanium dioxide layer without being blocked by moisture that would have been introduced by an intervening suppression step. Although the OMTFF of  FIG. 4  can exhibit a charge-eliminating function because more magnesium oxide and titanium monoxide are formed, the metallic composition (making the OMTFF  30  opaque to desired wavelengths of light) is increased. Consequently, this OMTFF cannot function as an infrared-blocking filter. 
         [0042]    The ability of an OMTFF to disperse electrical charges can be observed by conducting a simple charge-elimination test on a good-quality OMTFF  30  of this embodiment. The test can be performed by wiping the OMTFF  30  with a wiping cloth to charge the filter with more than 500 V static potential. If, when the surface of the OMTFF  30  is connected to ground, the static charge drops to 0 V, this indicates that the electric charge has been fully removed from the filter. If the same test is conducted on an OMTFF  30  after the first deposition step (without performing a suppression step and before performing the second deposition step), hundreds of volts are observed to remain on the filter  30  even after connecting the filter to ground, indicating that electric charge has not been completely eliminated. 
       Embodiment 2 
       [0043]    In this embodiment, the optical multilayer-film filter  30  of Embodiment 1 is utilized in an imaging module  100  of a digital still camera.  FIG. 5  is an optical diagram of the imaging module  100 . The imaging module  100  comprises a quartz crystal substrate  20  of an optical low-pass filter (OLPF). On the front surface of the optical low-pass filter  20  is the OMTFF  30 . Downstream is a solid-state image sensor, such as a CCD sensor or CMOS sensor. A driver  50  provides power and driving signals to the solid-state image sensor  40 . 
         [0044]    Incident light  70  enters the imaging module  100  through a lens  60 . The infrared radiation of the entering light is removed by the OMTFF as the filter transmits visible light and reflects the infrared light. By passage through the crystal substrate  20 , which serves as an OLPF, high-frequency components of the light that otherwise would cause moiré patterns are removed. The resulting processed incident light is received and converted to electrical signals by the solid-state image sensor  40 . The solid-state image sensor  40  is driven by the driver  50  to deliver electrical signals corresponding to the detected image to an image processor (not shown). The surface of the OMTFF  30  is connected to ground to eliminate static-charge accumulation on the piezoelectric crystal substrate  20 . 
         [0045]    In various embodiments, the OMTFF  30  is directly deposited on the crystal substrate of the OLPF. Alternatively, the OMTFF  30  can be deposited on another transparent substrate, such as a glass substrate. 
         [0046]    In various embodiments, titanium dioxide is used as a high-refractive-index material. Alternatively, other oxides such as tantalum pentoxide or niobium pentoxide, or oxide compounds in which these materials are in major proportion, can be used. 
         [0047]    In various embodiments, silicon dioxide is used as a low-refractive-index material. Alternatively, other oxides such as bismuth oxide, or oxide compounds of lanthanum oxide and aluminum oxide, can be used. 
         [0048]    In various embodiments, magnesium fluoride is used for the final layer. Alternatively, lithium fluoride can be used. Note that the atomic weight of lithium is 6.941.